Hi Lish - MoI does not yet have a "N-Sided patch" type tool which is what would normally used to fill in an area like this.

But in this particular case it looks like that is a trimmed out area, so like Petr mentioned the cleanest solution would be to restore the underlying surfaces there instead of creating a patch anyway.

To do this will require working with the surfaces individually. Start by selecting the object, and then use Edit / Separate to break it into individual surfaces.

Then for each surface around that hole, select one edge and then do Ctrl+A so that all the edges are selected. Then push Delete to remove the trimming boundary and recover the original underlying surface.

One complication is that the underlying surfaces are actually a bit larger than the original boundaries so the uncovered surfaces will need to be trimmed back down. But as a shortcut for 2 of them you can assign the "ShrinkTrimmedSrf" command to a shortcut key and run it on the 2 lower surfaces before untrimming.

That will leave just this one surface left to deal with:



After untrimming this one, you will see it sticks out to either side:



Trim that surface with the edge curves to either side and discard those ends, and then you can select everything and use Edit/Join to bring it back to a solid that has been fully restored:




I've attached the repaired model here as fill surface2.zip


There is a tutorial here on these kinds of object repair techniques:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

And also some more information here about untrimming:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

Hope this helps!

- Michael