To insert a Motion Blur:
1. Open Video BusTrack. (CTRL+SHIFT+B)
2. Right click the Video Bus Track Header and choose "insert envelope>Motion Blur amount.
3. Adjust the level of the motion blur. Because the motion blur is a very intense process, you'll want to do some short RAM renders to determine exactly how much you need. If you're using much motion blur, consider using a small amount of supersampling at the same time to keep it smooth in terms of how it draws the blurs. No system will display motion blur at high levels and show real-time frames, so doing the RAM render is fairly important. SHIFT+B is how you do a RAM render. Highlight a region, and press Shift+B.
HTH