How do i use frame picker?

You're probably not using a Graphic symbol. As said above it does not work with a Movie clip, only a Graphic Symbol which you can change by right clicking the symbol in the Library and selecting Properties. The workflow is basically:

• Place the symbol on the timeline...
• Select the symbol.
• Pick the Frame Picker and you'll see whatever you've placed inside that symbol. The symbol MUST be selected.
• Scrub your audio until you find where you want the mouth to change... tThe audio needs to be on Stream not Event which you can change in the Properties window when the audio is selected.
• Select that frame with the Selection tool (V)
• Create a keyframe by hitting F6
• Select the drawing you want in the frame picker and scrub to the next area you'd like the mouth to change and repeat the steps above.