It's hard to add frames. The best option you have is to drop your video into a 30 fps comp, enable frame blending or to interpret the footage as 24, 25, or 30 fps and live with the footage playing back a little faster.
I presume you see this taking place on a RAM Preview, right?
Your computer simply may not be able to keep up with the bit rate of the clip, especially if it's highly-compressed HD.
There are a bunch of ways to speed up previewing: reduce comp resolution to half or quarter, Preview every other frame, set a Region of Interest to limit how much of the comp is actually previewed... those are popular ones.