Wow, that's really unusual. I don't think I've ever seen a Martin without pins. Now that I think on it, I'm sure I've never seen any Martin without pins. Perhaps it was repaired at one point. Do you know how old it is? Does the finish look original? What kind of pegs does it have? Friction or geared? I think someone must have worked it and replaced the bridge. Too bad as Martins in their original condition are sought after. Anyway, tying on a bar bridge is really easy, and I prefer bar bridges far more than pin bridges anyway as I find that ties are more stable once the string is stretched and set. The knots required for pin bridges make the process of setting new strings longer and kind of a pain, especially if you make the mistake of making the knot too small, which can result in the string popping out from the pin groove. Ouch!
Anyway, if you've never tied on a bar bridge:
1. Push the string through the hole from the sound hole side about 3 inches.
2. Wrap the loose end around the string once, this creates a loop in the string
3. Wrap the loose end around the part of the string that makes the loop twice
4. Pull the loose end close to the body of the uke while pulling on the other end of the string
5. The loop part captures the lose end against the back of the bar bridge and makes the end lie down against the surface of the uke
6. trim
You're done!
Lot's of internet pics on the procedure.
Good luck!