I just wanted to help.. Because I just started using DH usb and its kinda cool especially if you dont have bigger MS. First, what version of DH are you using? I recommend DevHook v0.46 Tetra Pack. If you have that here's what you have to do.

1.After downloading dh 0.46 open the MS_ROOT folder and copy the 3 folder named DH,ISO,PSP to your memory stick and overwrite it with your exsisting one. Remove the usb cable to your psp after.
2. Then go back to your downloaded dh 0.46 and you will see a folder named PC. Just open and open it and you will see ISO folder(you need to put your game ISO's or CSO's here) and a PSP Type B Driver folder. Open the folder and install the program named (PSP Type B Driver install - Multi Language)
3. You can now connect the usb to your psp. Turn your psp On and run devhook emulated firmware launcher 0.45 configuration menu.
4. Browse through the menu and you will see "OTHER" option, open it and enable the usb option.
5 Your computer will say "found new hardware" just install it by choosing the automatic option.
6 After that, when you go to UMD SELECT you should see now the game ISO's you copied to you ISO folder in you harddrive.