According to
this page, the PowerMid is a remote control extender. You put one half in one room (say your kitchen), and the other half in the room with the equipment you want to control (say your living room). The first half takes the IR from your remote, converts it to RF, send it to the second half when then takes the RF and converts it back to IR. That's not what you're looking for.
The IR543 is not an IR to RF converter - it is a IR to X10 converter. The difference is it will directly control your X10 devices by plugging into the wall. With an IR to RF converter you'd still need an RF to X10 converter to actually control your lights.
The IR543 is the original manufacturer's part number. It is rebranded into many different packages and names, but as far as I know it is the
only IR to X10 converter on the market -- and it's what you're looking for. It's about $30 and is a small low black box with white buttons on top for direct control. The one I saw was about 4" wide, 3" deep and 1" high (from memory). If you see something matching this description, it's it.