Select the curve and open the IPO window and choose ‘Path’ from the drop down, may be on ‘Object’ by default, least mine is.
Ctrl+Click at the frame you want the camera to start following, make sure Vertex Y: is at 0. Then Ctrl+Click again at the frame you want the camera to complete the path, only this time make sure Vertex Y: is at 1.
Edit: May also be useful to know that you can cause the camera to pause on the path by straightening out the IPO curve somewhere.
Example: First IPO vertex is X:1, Y:0, so the camera starts following the path at frame 1. Second IPO vertex is X:25, Y:.5. Third vertex is X:75, Y:.5. This causes the camera to stop half way through the path for 50 frames. Last vertex X:100, Y:1, which causes the camera to complete the path over 25 frames.
Maybe that’s confusing, heh.