For really long pieces, the previously suggested methods are the ticket. If you are going under 3 or 4 feet along the dado and less than a few feet from an edge, just cutting an offset board seems like the best to me. If explanation is required, see below.
Get the offset of leading or trailing edge of the bit from the edge of the router (should be a reasonably round number since the router base and bit size should be a somewhat round number) and rip a piece of plywood to the correct width to offset the router the correct amount. Then, place both the offsetter and the piece to be routed snug against a fence and clamp with the offsetter on top. You can cut an offset for the piece you are routing as well if necessary to get enough offset (for dados close to the edge), and lay it next to the piece instead of on top of it, then use the fence itself to run the router against. Or you can measure from the top instead of the bottom and flip the piece around. I have tried various ways of accomplishing what you are trying to. I also have the Porter Cable attachment you linked to. I use it sometimes, and it is nice, but a cut piece of 3/4 plywood is very repeatable, never changes, and I feel like I have more control riding the router along it. Also, quite cheap, especially if you have plywood scraps laying around.
So, leading edge of router bit is 2 5/8" from edge of router. You want the dado to start 1/2" in from the board edge. You can cut a single board 2 1/8", lay it next to the work piece so it is set up as fence, then spacer, then workpiece. Clamp. Run the router along the fence. For larger offsets eventually the spacer moves to the top of the piece and both go against fence. It only gets a little silly if you are trying it for pieces 3 or 4 feet long with a dado down the middle.
I know that is overly wordy, but there it is.