Bet,
I had the same problem when I first installed the maximized version. If youre up to it, I can tell you how to edit the xml file to fix the problem. It's very simple. This is how I fixed the problem on my end. I'll give you a list of steps to make it super simple.
1. Go to your Media Center directory, then MegaSkins, then into Codename Opus.
2. Right click on the main.xml file and choose edit. Dont worry about backing it up because if you mess it up you can just reinstall the skin.
3. To make it easier to read, maximize the window.
4. Right near the top there should be a section labeled like this:
<MAINFRAME>
blah
blah
blah
</MAINFRAME>
5. On line 11, in the middle of that section, there should be a line labeled "Closebutton"
6. At the end of that line, it should say OffsetX and OffsetY
7. This is exactly what those variables should look like:
<Entry Name="CloseButton" Bitmap="MFCB.png" NumberImages="3" Alignment="2" OffsetX="0" OffsetY="0" />
That should fix your problem, or at least, thats how I fixed the same problem on my system about 2 days ago when I installed the maximized version and had no close button.