Ah, your lens doesn't have a focus scale. If the moon is in sight just focus on it's edge and set the focus switch to MF.
I found a review of your telephoto lens. A few pages in it has a useful graph identifying areas of poor performance. 300 mm wide open at far focus is especially bad.
http://194.100.88.243/petteri/pont/R..._f4.5-5.6.html
Since astronomy forums tend to be pretty snobby with regards to image quality, I feel that its not out of place to mention that the 90-300 is a bit like the 60 mm Tasco of the SLR lens world, although at 90 mm fl it apparently isn't bad.