Manually - I.E either mount system r/w with something like Remount from GP, then install the apk, or drop the apk in system/app & set permissions (either in shell or with Root Explorer/similar) to 644 (rw-r-r) ?
Might not work/have problems with recent versions of Flash though, not supported officially on Android anymore & on a 3 yr old device you're not guaranteed to get it working.
Edit - If you use terminal to do it, don't forget to switch to root first.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?