The basic idea is that to use a 64-bit version of Eclipse, you must have a 64-bit JVM and an operating system that will support 64-bit applications.
When in doubt, choose the 64-bit version. However, one reason to chooose a 32-bit version would be if you are interfacing to a 32-bit version of Dymola using FMI.
http://windows.microsoft.com/en-US/windows7/32-bit-and-64-bit-Windows-frequently-asked-questions. for details.
Return to the Installing Eclipse page.