Search for an application called "Configure Wine" or "winecfg" (I'm not sure exactly what search term will get you to it in Unity because I'm using 10.04 with Gnome 2. You can start it from a terminal with "winecfg". If you're using Gnome fallback mode in 12.04 instead of Unity, you should be able to start it with Applications -> Wine -> Configure Wine).
Anyways, once you've started winecfg, go to the "Drives" tab. There will be a section marked drive mappings, which should look something like this:
What drive letters and mappings do you have listed?
Letter Drive Mapping
Also, more recent versions of X-Plane have native Linux versions. You might want to try the demo for X-Plane 10, and, if it works well on your machine, buy it. (I have the Linux version of X-Plane 9).