I had similar problems with the TeXlipse spellchecker. To fix it I followed the instructions from https://stackoverflow.com/a/5736974/2920749 Additionally, I set the language code to en in the project settings as described here: https://stackoverflow.com/a/7064110/2920749. However, spellchecking still did not work for me under Ubuntu 12.04. I had to switch to a monospaced font to get it working.
Also make sure that the dictionaries are uncompressed. Check that all dictionary files are readable and that you have write access to the user-defined dictionary.