did you read the read me file, because it tells you how to use the trainer
click SAFEHOUSE in main menu
you either are getting out of VAN or standing in street
press XP option. hear activated. each press adds 1000 XP
press ESC key to open menu
press LEAVE MISSION or END mission or whatever to go back to main menu.
XP is increased and likely your level has also.
you don't need to use the computer or anything else. If this doesn't work for you then I have no other suggestions as this works 100% every time here.