I recently got Motorola MPx200. I tried to download java games and after i download the game into desktop PC it has a file extension .jar. How do I install that file into my mobile?

does your phone take .Jar file? ....although this IS the game file, you may need a .Jad file to load to phone, a Jadgen (free tool) will create this from the .Jar
MSS will (i think) load a .jar ...but this can be dodgy to use as it is capable on making it a paperweight with the powerful uses available.

Midway is the favoured tool for cable users, this loads the .jad