To install any "Minecraft PSP" package (including the 321 FATZIP), your PSP must be running custom firmware (CFW). If your PSP is stock, you cannot run homebrew.

Zip. Unzip. Install.

Elias stared at the filename on his desktop: minecraft_psp_321_fatzip.zip .

To advance this conversation, would you like a step-by-step for PSP homebrew, or