@pescadito
Aarrgghh - sorry, i forgot:
the SD750 is the one with the corrupt +/- bracketing 
You've done it all right; the CHDK Tv bracketing feature is not working correctly on this camera, so you need a script instead...
There's a script in the german forum that should work, i've attached it to this post (translated, but not tested)...
- store the file "bra_uni.lua" in the \CHDK\SCRIPTS folder & load it from the CHDK script menu;
- you can set up some parameters there in the script menu...
- start the script with the shutter button
I guess there are also some working bracketing scripts in the CHDK wikia...