The software often requires a "base" PBP file (usually a small official PSN game) to function as a template for conversions.
When you open the software, you can choose between "Theme Mode" (visual-heavy) or "Classic Mode" (a simple, functional window). Most veterans prefer Classic Mode for its speed. Selecting Files: Input ISO/PBP: Select your PS1 game file. psx2psp 1.4.2