Not sure why you want the old version, remove it from your machine. Do this AppCleaner (free to download). You can't be "signed on" to ACOM from both programs. You're confusing you're machine.
If you have tree files in the old (FTMM2010) program, you can easily port them into the new version. You do this BEFORE you trash the old version files.
Make sure you're SIGNED-OFF of ACOM in both programs before porting the files from old to new version.
Once the old version is removed, reboot your machine, then open FTM for Mac 3, sign on to your ACOM account and proceed to download the trees.