Here's some random thoughts about the Jena 6 slogans:

"Free [the] Jena 6" means "release them from jail {or prison}".

"Free ___" has been used many times by African American groups and other groups protesting what they consider to be the unjust incarceration of an individual or individuals. See the reference to "free the panthers" in my 23 Sep 07 - 10:00 PM post to this thread. Also see the African American political blog http://www.colorofchange.org/campaigns.html for other examples of "free ___" mobilizations efforts.

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"No Justice. No Peace"
This slogan means "Until there is justice, people will agitate for changes and there will be no peace."

Or it means "Until there is justice, people will raise a rukus and there will be no peace."

Or it means if there is no justice [for all], there will be no peace [around here].

In any event, the succient alliterative pattern of the word "no" + a desired condition and then the second "no" + a desired condition is what makes this slogan so powerful.

I think that the people who chanted or who created the signage "no justice, no peace, no racist police" got the alliteration part of this slogan, but didn't get that the first two conditions are desired but the third one is definitely not desired.

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"Blacks Protests N'Justice"   
I like how hip-hop spelling has worked its way into slogan writing.
"N'Justice" of course means "injustice". With what I call "hip hop languaging" it's not just how a word is pronounced, it's how it looks. Punctuation marks and clipped letters enhance the look of a word {and personal names} while retaining the pronunciation, and, in some cases, getting even closer to how a word is actually pronounced.
Since the first hip hop record came out in the late 1970s, slogans using hip hop stylin is another way that African American protest slogans have changed since the mid 1960s.

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"They stood for us now we stand for them
Free Jena 6"

With the reminder that this thread is not meant to be a forum for the discussion of the Jena 6 case, the "stand up/stood up" phrasing of this slogan reminds me of the Christian song Stand Up, Stand Up, for Jesus. I'm particularly referring to both the title, the refrain, and this verse:

Stand up, stand up for Jesus,
The trumpet call obey;
Forth to the mighty conflict,
In this His glorious day!
Ye that are men, now serve Him
Against unnumbered foes;
Let courage rise with danger
And strength to strength oppose.

-snip-

Again, some will debate whether the Jena 6 as a group or as individuals stood up in the face of unnumbered foes or whether their cause [of unequal treatment within the law] should be supported {that is that people should "stand up for them"]. However, again there's another Mudcat thread for that discussion.

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"non-violence or non-existence"

This is another slogan that uses alliteration {or whatever this literary pattern is called}. Imo, it's not as effective or powerful as the "No Justice. No Peace" slogan, perhaps because of the circumstances of the Jena 6 case, or just because I'm not sure what is meant by the slogan. I suppose the people who held that sign or had that sign on their tee shirts meant something like "Give me liberty or give me death". Now there's an effective, powerful slogan...

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"Equality"
I believe that "Equality" is a shorthand way of saying "equality under the law".

It's interesting to me that in the mid 1960s, "Freedom Now!" was probably the most widely used chant in the African American civil rights movement. The word "freedom" worked on many levels since that word harkened back to African Americans' emancipation from slavery and that word also expressed the desire, nay the demand for Black folks and other people of color to be free from racial oppression.

But it seems to me that "freedom" is too abstract a concept in the 21st century. I think that Equality [under the law] and "equal treatment of individuals and groups by those in authority" is what we should be striving for rather than Freedom.

Or maybe "Equality!" is just the modern way of saying "Freedom!"