Can you be a little more precise? Are you talking about doing something in Captivate Edit mode or at runtime?
If you're talking about changing the frame-rate of a Captivate project, then go to Preferences > Project > Project Settings > Frames Per Second.
If you are talking about pausing the playback at runtime, click the Pause button on the playback bar. There is no button option on playbars to slow down the playback, only Pause, Play, Back, Next, and Fast Forward.