1.) Click the arm of your character. Goto Insert > Convert to Symbol.

2.) Now choose Movie Clip, and name your characters symbol arm. Click ok. Then insert keyframe on 5 and 10 then select all the frames, right click and Create Motion Tween.

3.) On frame 5, move your arm and rotate it a bit, just like I did above (the left pic). Here is where it gets a little tricky, but its how I do it. Now on frame 9 Insert Keyframe, then on frame 10 Remove Frame (the right pic).

4.) Publish your animation and now you got a character moving his/her arm and if you did the last lesson you have him/her blinking as well. Next lesson you will learn how to make your character walk!