The installer for older versions gave you the option to install both 32 and 64bit versions of 3ds Max on 64bit Windows.
The new installer installs only the appropriate version of 3ds Max for the operating you're installing on:
- the 32bit version of 3ds Max 2012 on 32bit Windows and
- the 64bit version of 3ds Max 2012 on 64bit Windows.
To install the 32bit version you need to run Setup32.exe which is available in the root of the media or the temp folder which is created when you run Autodesk_3ds_Max_2012_English_Win_32-64bit.exe (the default is C:\\Autodesk\\Autodesk_3ds_Max_2012_English_Win_32-64bit\\).
For additional installation related tips read the Installation Help