You have various ways to activate PSN games on CFW 4.90
1- Install the PKG from PSN Store. Your game will be activated automatically when the download is started.
2- Install the PKG from a storage device. Then go to PSN Store and start to download the same game. After the game is activated, you can cancel the download.
3- Create a new user account, use CFW Tools menu to activate it with a fake account. Copy the .rap files to /dev_hdd0/exdata or /dev_usb000/exdata or /dev_usb001/exdata. Cobra will activate the game automatically when the game is launched.
Some homebrews also allow activate games: Apollo Save Tool, PSNpatch or reActPSN (I'm not sure if it's still compatible with 4.90)