How to add face to this object? getting strange results

Greetings,
Im having problems adding a face to this area. Its a cylinder in a diamond shape that I want to keep the round base but add a face to the squared center, when I select and F it joins but has a strange bend instead of a straight plane. is it possible?

[ATTACH=CONFIG]422013[/ATTACH]

[ATTACH]422012[/ATTACH]

you have lots of internal geometry, which should be deleted, also some doubles, that can be fixed with ‘remove doubles’ on the tool shelf. a couple other minor problems, such as normals facing the wrong direction, which can be fixed with ctrl N, but only after you have removed the internal geometry. I am not sure about your question, I think I was focusing on the wrong area, but it might be a non-uniform quad causing the problem. ( non-uniform means it’s bent instead of flat-ish. )

You do not have used Mirror modifier at it’s all. If you had you’d have to model just 1/4 of your object and deal with 4 times less vertices to create and move around.
To use mirror on X and Y you need center of symmetry to be at coordinate grid center; right now it’s off. Shift-S, cursor to center, Origin to 3d Cursor will correct this. Switch off Mirror modifier visibility (eye icon on modifier) and scale selections of vertices which should be on x and y axes to 0. If mirrored edges are not yet connecting leaving gaps move them closer; Clipping on Mirror modifier should be on.

As Modron says, there are a bunch of ‘internal’ faces which make object look ugly when seen in 3d view solid or rendered views. You need only the ‘shell’ faces. Select and delete all which you can not see from ‘outside’ is a first thing to do.

For a simple model you could leave cylindrical part separated, even separate object will do; if you want to be perfect use Intersect, option Self Intersect on selected faces to get intersection edges. Delete not needed faces after.

If vertex count is not a big concern (as i see on a rounded object) add some edge bevels Ctrl-B on sharp edges, this helps shading and overall look. (You could use Decimate modifier, Un-Subdivide, 2 iterations easily to get less vertices on that round part. Apply Decimate modifier. Again, use Mirror.)

http://www.pasteall.org/pic/show.php?id=98708
http://www.pasteall.org/blend/40452