This is just in case anyone has the same problem I had.
My APC20 wouldn't Auto or Pre-Map. I connected the device and set my midi preferences correctly;
Input / Output - APC20, / Track, Sync, Remote turned to On.
The midi indicator on the top right acknowledged a signal from the device but no red lines appeared around the cells in session view, nothing would light up on the APC20 when I dragged and dropped a track into the corresponding clip on Ableton, the Volume control didnt do anything etc. I was able to manually map the APC20 but that's exactly what I didn't want to have to do.
I tried everything I could think of to fix this, re-installed ableton, different versions, different USB ports, re-starting the computer, turning the APC20 on a different times during start up etc, messed around with the midi preferences but nothing would work!
If your having the same problem are you running WINDOWS VISTA? If so your out of luck, the only way i got everything working was by upgrading to Windows 7! Now everything works perfectly
Cant help you with how to get it to work on Vista I'm afraid but hopefully this will save some of you the pain I went through. Nothing is quite as boring as sitting about reinstalling things with no luck.