The theory would be you select the clip in the project bin, make a copy if you have audio, then right-mouse click (Modify | Interpret Footage — that’s how it looks in CS5). Set the frame rate to 30. Then you drop that in a sequence (you can use the New Sequence from Clip menu) and it will play at half the speed in Premiere (be half as long), you then make another new sequence from the first sequence and apply Twixtor there with 50% speed. If you have audio, you might have to turn off audio from the retimed one and put under it the not retime one and turn off the video part for that one (just keep the audio tracks).
Now you say it’s 15 FPS, it might be 14.985 FPS in a 29.970 FPS sequence, if you have problem with that, we have techsupport @ revisionfx dot com here to help out.
Also there are some tutorial intro about Twixtor in general here:
https://library.creativecow.net/tutorials/revisionfx
start with overview for premiere pro (they are in inverse chronological order) OR here help.revisionfx.com
Pierre
RE:Vision Effects