In the monkey 64 folder when you download it, there will be a 1.5 or 1.0 folder. If you haev 1.0 or 1.5 then click on one of them.
To find out what version you have:
http://www.pspbrew.com/forum/viewtopic.php?t=3114
If you have higher than 1.5 or 1.0 then you are currently screwed since there are no eloaders (I think) that run N64 emulators right now.
So I reccomend you wait if you have a higher firmware.
But if you have version 1.0 or 1.5, all you gotta d it go in that 1. or 1.5 folder int he Monkey 64 folder you downloaded (find it
here
then look in the 1.5 or 1.0 folder to find a PSP folder. In the PSP folder you will find a GAME folder. And in the GAME folder you will find two folders. Copy and paste those two folders in your memorsy stick of your PSP (go into USB connection and connect your PSP USB cable to your PC) and you will be in the ROOT of your memory stick.
If you have not all ready, create a PSP folder, then create a GAME folder, and paste those folders of the PS Monkey 64 emulator you copied earlier into the GAME folder.
You are done! Now just go into the GAME section of your PSP and you will see the Monkey64 emulator! Just place the roms inside the folder without the % sign and you will be able to play cool 64 games!