I solved this problem. When you uninstall MGLTools 1.5.6 rc1 (latest nightly build), you have to remove MGLTools directory from
2. Remove MGLTools folder from MacintoshHD/Library/MGLTools
now the "tricky" part:
MGLTools installer on MAC OSX 10.6.4 adds several invisible files to the Macintosh HD drive. These two files - MGLTools.bom and MGLTools.plist are located in MacintoshHD/Private/var/db/receipts. You dont see this folder because it is a hidden folder. You have to remove this two files too.
Then you can install an older version of MGLTools etc 1.5.4 on your MAC
Thanks Alexey! This would be very useful for other Mac users as well.