You will need the 64 bit version of the plug-in if you are installing the software in a 64 bit machine on which you have installed a 64 bit version of a CAD program. The bit version of the plug-in corresponds with the bit version of the CAD program, not the machine.
e-SPECS is a 32 bit application and when it's installed on a 64 bit machine, the DSN it needs to connect to the SQL database is automatically created in C:\WINDOWS\SysWOW64\odbcad32.exe
CAD programs like Revit Architecture 2013 and AutoCAD 2013 come in both 32 and 64 bit versions, either of which can be installed on a 64 bit machine.
If you install the 32 bit version of a CAD program on a 64 bit machine, it uses the DSN created during the installation of the e-SPECS software in SysWOW64 to access the SQL database.
If you install the 64 bit version of a CAD program on a 64 bit machine, you will need to create a second DSN in the usual place: Control Panel > Administrative Tools > Data Sources (ODBC ) > System DSN tab. Configure it to be identical to the DSN in C:\WINDOWS\SysWOW64\odbcad32.exe.
Additional instructions for manually creating a DSN can be found in the e-SPECS for Revit Installation Guide under Configuring the e-SPECS DSN.