Hi'
I just saw your post and know how you feel.
I had a similar problem and while I failed the first time that I tried to load my new tablature, I suceeded the second (or maybe 5th time) by carefully following the instruction on pages 38-39 of the manual.

With the USB stick inserted, Press and hold the "SET" register until it flashes slowly.
Press button "37" on treble keyboard in order to set the "IMPORT TABLATURES" parameter.
The (SET) register should now be flashing quickly, and registers 1, 2, 3,should be lighting yellow or red.
If any one of them is yellow. This means that the FR18 has detected a new file, but if they are all red, then it means that there is nothing new detected?
(At this stage, try resetting the FR18 to it's "Factory default" settings.)
If there is a register that is lit Yellow, then you press that one and you select the "Internal Set memory" (Register 1, 2, or 3.) to which you want to load the new tablature.
Now all the registers lights should have gone dark.
Press the "SET" register to exit the function mode.

Good luck.