J2ME games ship as .jar files (sometimes with a companion .jad descriptor — you only need the .jar). Import the file and Handyplay reads the midlet name, vendor and icon from its manifest.
Screen sizes
Midlets were built for specific screen sizes (128×128, 176×208, 240×320 were common). Handyplay picks a sensible size for the game and scales it to your screen; if a title renders at the wrong size, check its entry on the Games pages for known-good variants.