Daisy,

The short answer is that you can't stick diminished chords into your old repertoire.

The chords to the arrangements to your tunes don't change because you've got new buttons one your accordion. What you have to do is learn some songs that include diminished chords in the arrangement--someone mentioned, above, that many popular songs written in the last part of the last century and the first six or so decades of this one feature diminshed chords. Check out a songbook that features materials from this time period and you will come across some.

Of course, it is possible to write completely new arrangements to the songs you know, and to include the diminished chord sound, but that takes a bit of music theory--

Here is something you can try, just to give you as taste:

Take a song that you know, in say, the key of C--replace all the G7's with a Bdim

(note: Bdim is the same chord as Ddim,Fdim, and Abdim, so you can interchange them, or play them in scale order--Bdim-Ddim-Fdim-Abdim-Bdim, to get the "Young-girl-tied-to-the-railroad-tracks" effect. Your friends and family will get sick of this trick a lot sooner than you will, though, so be warned.)