All you have to do it upgrade to 4.01m33-2 OR 5.00m33--2-3-4-5 or 6 either one works fine.
If your on 5.00m33-2-3-4-5 or 6 then you use this popsloader: Free File Hosting Made Simple - MediaFire
just put the seplugins folder in the root of your ms.
If your on 4.01m33-2 then you use this one: Send big files the easy way. Files too large for email attachments? No problem!
When you are done with that go to recovery menu -> plug ins and just enable it.
(to get to the recovery menu you first must fully shut off you psp and turn it back on while holding the R button)
Remember when u have psx games they must be a eboot file.
Put them in your GAME folder and then you can enjoy ur not so new psx games ONE more thing i forgot to write down...
if your using 5.00m33-2-3-4-5 or 6 then before the psx game starts hold down the R trigger and there is going to be list of firmwares just pick 3.71. If your on 4.01m33 then choose 3.71.
3.71 works best (For some games as killer3737 said)
Note: You can only access the recovery menu if your psp is modded with custom firmware.