.jar files are Java, so unlikely to work. I went to the site and when downloading it asks for your phone manufacturer and model number. It looks like they've automatically populated that list with a cut-and-paste from somewhere, and when you pick N800 or N810 it just suggests to "try" the latest Java version, without actually saying whether it will work or not.
You could install it on your phone and save the nit's battery. This site has it, and lots of other free java games. First click on "set mobile" then click on "More Games" in the "Java Games" box and you'll have lots of games. You must try CrazyBall, The Entity, and 5udoku if you loke java games. Gladiator Manager is pretty cool too.
