MrSid files are not available as a background layer type or an error is generated that says " "MrSID files are only supported when running the 32bit version of the application."
MrSid files are no longer supported starting with the CONNECT Edition Update 1 releases (10.01.XX.XX) for technical reasons because of the introduction of Bing Maps.
In earlier versions, MrSID files were only supported in the 32-bit version of the OpenFlows application. When you open the application from the shortcut on the desktop of your computer, it will be opening the 64-bit version by default (on a 64-bit Operating System).
If you need to use the MrSID file as the background file (in a version below CONNECT Edition Update 1), browse to C:\Program Files (x86)\Bentley\(program name) and open the file program executable file, such as "SewerGEMS.exe" for SewerGEMS. This will be the 32-bit version of the program and you will be able to use the MrSID file as a background.
The alternative would be to convert the .SID file to another supported file type or consider Bing Maps.
What file types are supported for background layers in the standalone platform?