How can I close this?

 From:  Michael Gibson
9361.7 In reply to 9361.5 
Hi Lee, thanks for posting the file. So in your model one side of the hole area does not appear to be entirely planar. So I think that's why an extrusion from one side to the other is not joining in well.

If you use something like a Loft between the 2 sides that should join in ok in this case.

It's also possible to fill in holes like this by removing trimming boundaries, there is some information on doing an "untrim" here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

And there is a tutorial on this kind of process here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

- Michael