In the Aircraft tab, just make sure you have the correct weights (and correct units!) entered. Also, assuming you use Active Sky (Any version really) make sure it's running and connected to FSB (using the FSB options menu) to give you the enroute winds. A good way to check is to open the navlog and look for the winds entries. If they're all 00000, then it didn't get the data.

Thanks, i'll give it a go.

Try here - http://forum.avsim.net/forum/153-the-flight-plan-forum/