Create two new layers in the timeline. Call one 'label' and call the other 'code'
Go to frame 75 and click in the 'label' timeline. Press F7 to add a blank keyframe. Goto the 'properties' panel and look in the 'label' section for the 'Name:' box. Type 'loop' (without the quotes) into that box.
Goto frame 100 on the timeline and click on the 'code' layer. Press F7 to add a new blank keyframe. Go to the 'Actions' panel and (with the 'script assist' button off) type the code
gotoAndPlay("loop");
Press Ctrl+enter to test your movie.