HELP with installing games on phone

ok well i'm kinda a noob with this, but i got a nokia 6230 phone and i want to upload games onto it. i downloaded some files from nokiapower.com and they came to me as .zip files. well using the nokia PC suite, the files gotta be in .jar format before they can get uploaded. So i used winzip to just erase the .zip portion of the name and renamed it to .jar and then i uploaded it to my phone. but when i check on my phone the application names appear in grey and i can't access them. when i select them my phone outputs a msg saying 'invalid application'......sooo what i'd like to know is what i'm doing wrong.
if anyone could help me out, i'd appreciate it

thanks

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just changing the filename wont make your file into that file type. you have to convert it first.