First of all, I misread your post. I thought you were getting this error when trying to install SeaMonkey itself... My mistake.
Most extensions written for the Mozilla suite should work in SeaMonkey. The unofficial site
seamonkey.be has a quite extensive list of extensions that are known to work with SeaMonkey.
I don't know if this is any help, but were you trying to install extensions written for Firefox? If you were then that could cause problems. Look for a version written for SeaMonkey (or the Mozilla suite, or Mozilla 1.x).