hav'ing problem with my new phone....

i just got a new phone frm my mum, nokia e65.but then actually im in love with the nokia navigator in pearl white. eekekekekeke but im ok as long as got new phone to use. but then, whenever im trying to install a new themes...it shows something like 'not certified application' something like that la. haih so i got no nice theme to use now....wtf. the file is correct, sis.file but why cant? can anyone help me wit that?? :(