1. Using the cable trim/tensioner loosen the cable until it's slack.
(If you can't get it slack, then the cable was bound to the FD while the shifter was not set for the small chain ring position. You'll have to disconnect the wire in this case. Go to step 2.)

2. Check to see if the shifter will shift up and down properly by holding the slack inner wire in your hand and applying tension to it. You'll feel the shifter pull for two shifts and release for two shifts, if the shifter is OK. Reconnect the wire to the FD while the shifter is in the position for the small chain ring if you disconnected it in step 1.

3. If the shifter is working, leave it in the position used for the small chain ring, then tighten up the inner wire using the trim/cable tensioner until there's no slack, but not overly tight. If the shifter is not working while the wire is loose, then see your LBS.

4. Try shifting up one position. If it works, leave it alone--you are [probably] done--try step 7. If it doesn't work, grab the inner wire and pull with enough tension to get to the center chain ring. Hold the wire in that position with one hand and with the other hand tighten up the trim/cable tensioner.

5. Shift back down to the small chain ring.

6. Redo step 4 and 5 until it shifts properly between those two positions.

7. Try shifting to all three chain rings. Use the same technique as in steps 4 and 5 to get all three shifts to work.

If none of the above works (or you just don't feel like futzing with it), go see your LBS.