alyssamy,

Without seeing your results it's hard to say, but a possibility is that non-smoothness is a consequence of how you took your photographs. Two factors top the list: If you didn't use a tripod the backgrounds will be unsteady. Pro film makers can use very expensive programmable camera supports so that they can duplicate pan and tracking shots but you'd do best by shooting from a rock steady position for each scene. The second is the spacing of your shots. If you check through you film using the arrow keys you might spot places where the recorded movement between two frames is too much (you skipped a shot) or to little (can maybe be corrected a bit by having one image appear for two frames instead of one).

YR suggests it's a performance issue which is likely if you have a very large number of high resolution stills. An option there is to use a technique called 'frame serving' where a utility program (free VirtualDub) is given your stills and it creats a file that MEP thinks is a simple video clip. Is you mivie any smoother if you are working with only a few seconds of acton? You can aways render lots of sections and join them together later. 

Hope this helps,

Carl