1.5.5 is really old.
The 1.3 jar will only work with version 4.0 of SQL Developer.
You can get the 1.2 jar - i think that will work with 1.5, or you can get a newer version of SQLDev off of OTN and use the 1.3 JAR. I very much so recommend the latter.
Thanks very much! I was beating my head against this one for far too long. Appreciate your help.