That file is a description to a java capable phone, it does not actually include the application. You need a .jar file to run on the N900.

In this case, the jar file to go with that .jad is: MBSN_L1.0R1H_b9.jar
Do you have this file? If so, you simply need to copy that to your phone (anywhere really, it doesn't matter) and then select it from within MicroEmulator.

If you do not have this file, then you should be able to download it from the same place where you got the .jad file, by replacing the .jad file part at the end of the web address with this:
MBSN_L1.0R1H_b9.jar