On modern custom firmware (CFW) like Evilnat , you can simply place the .dkey file in the same folder as your ISO. Tools like webMAN MOD will use it to decrypt the game "on-the-fly" while mounting, making the process seamless.
Comprehensive Technical Report on the PlayStation 3 "dkey" (EID Root Key)
: The core tool, PS3Dec.exe , is command-line based, which may be intimidating for beginners without a GUI front-end. Where to Get Them
There are two main ways to use these keys depending on whether you want to decrypt the game permanently on your PC or "on-the-fly" on your PS3. Method A: Permanent Decryption (on PC)