I found the best way to put the seats in was - hang the front allen (hexagon) bolts through their correct holes in the seat rails with the seats at the back end of their travel and line them up over the captive nuts before loosely attaching. Then push the seats forward on the rails to get the allen bolts in at the back. These ones are easy enough to get pretty tight using a normal allen key.
The front ones are hard to access though and I used the above 'multi-angled' key with a ball type end.