iMacros includes a 32-bit and 64-bit version of the Scripting Interface. The x64-interface allows you to run iMacros directly from any 64-bit application on Windows x64 systems. For example, you can use it with ASP.NET with IIS running in 64-bit mode.
The use of the 64-bit version requires no code changes. After you install the 64-bit version of the Scripting Interface, 64-bit applications automatically access the 64-bit version of the interface. 32-bit application continue to work on the same system, as they automatically continue use the 32-bit version of the Scripting Interface.
Please, notice that there is no x64 version of Microsoft's ODBC Driver (e.g. to connect to an Access database or use the "Microsoft Text Driver" to query a file). If your application needs to connect to this driver you will have to compile it against x86.
Note about VBS scripts: If you start a VBS script from a Desktop shortcut or Windows Explorer (= 64-bit applications on 64-bit Windows!) the VBS script runs in 64-bit mode. If you need to run the VBS script in 32-bit mode, you will have to use the command line %windir%\SysWOW64\wscript.exe yourScript.vbs, which calls the x86 counterpart of wscript.exe.
V6.65+: The last two digits of the returned version number of iimGetInterfaceVersion() indicate if you are using the 32bit or x64 Scripting Interface. Example: 32bit: 80068832, 64bit: 80068864
Related note: iMacros for Internet Explorer is not available for the 64-bit version of Internet Explorer. iMacros runs always in the 32-bit version of Internet Explorer. So it works on x64 system in the 32-bit version of IE.