Tools > Options > Library & Folders > Manage Library Fields.
Find "Description" in the list on the left, select it, then look for the "Default Search Field" check box in the lower right. Mine is selected already, though I cannot remember if that is a default setting or a change I made a long time ago.
The "Search Keywords" field allows you to add a short text pointer, which I'll explain below:
In the search field in the top right of the program, you can just enter some plain text, and MC will search. It will include all fields that have that "Default Search Field" setting checked.
If you wanted to only search in the Description field, you would type: [description]=search
If you added a search keyword to the field properties, lets say you chose, desc, then instead of typing the string above, you could type desc=search and get the same results.
I have things such as:
[filename (name)] - fnn
[filename (path)] -fnp
[album artist (auto)] - aaa
aaa=bob is a lot easier to type than [album artist (auto)]=bob, you see?
-marko.