I have been so frustrated for the past year with Microsoft Word 2010. Every time I type a medical word into a document (like, hepatosplenomegaly), I'd get the stupid red, squiggly line. I know the word is spelled correctly, but Word doesn't know that. I finally found the solution, or at least a help at MT Herald! I downloaded the zip file, took forever to find where to copy it to (for Windows 7 and Microsoft 2010 users it would be C:\users\yourname\AppData\Roaming\Microsoft\UProof) and voila! I had a medical dictionary in my MS word... the document I was working on was a lot less red! Oh, and it was FREE!
So, if you're in a profession that uses words that the general population doesn't use, find yourself a custom dictionary. I had to add a custom dictionary (Spanish) to my Firefox browser so I could type words like quesadilla and be able to spell check it (of course, I have to change my language for a minute, and then all of my English words are red, lol!).
**UPDATE 1/20/2014: I have since upgraded to Microsoft Word 2013 and the dictionary still seems to be working in my new program! I'm not sure where you would install this to with Word 2013, but it seems to be compatible with it.