you don't need to export a scene if you want to import the full scene back in.

File>Import> sellect dropdown ma or mb depending on what you saved it as.

If you want to export just 1 item from the scene, sellect your object
File> Export sellect box the sellect mb or ma

Then to import follow step one.

btw it's a very easy methode and should be in your help menu press F1 it covers alot of information.