Actually the E-P1/E-P2 are different in operation in this regard than the E-PL1/E-PL2 as they have no dedicated button for manual focus assist. Instead it is a menu-driven option. I own an E-PL1 so I'm afraid I can't be of any more assistance than that. Perhaps someone with and E-P1 will be willing to post the steps required to activate MF assist with legacy lenses.
On the e-p1, you have to make sure the 'magnifying screen' is in the live view screen cycle (menu gear *D->info->lv-info) ; then, you have to go to that screen (the one with a central green square) with the 'info' button when you want to take a picture and hit 'ok' to enter magnify mode and 'ok' again to get out.
On the EP1 to get the magnifying glass you need a firmware update , if you have bought it quite a while ago similarly for the GF1 , G1 comes in standard with the magnifying glass. Firmware upgrade all in all takes 2 minutes.
For the EP1 you need to go on the olympus site get the software install it on your pc connect your Pen ep1 to the PC and follow the instruction , BEFORE TRYING READ THE HOW TO OTHERWISE YOU WILL BRICK YOUR CAMERA.