The default English dictionary bundled with Eclipse is a bit weak, missing many words that are then wrongly marked as errors. I would thus like to change to a user dictionary; this seems to be possible in the dialogue Window > Preferences > General > Text Editors > Spelling.
I have tried providing the paths to the .gz and .rws files found in the aspell folders, but Eclipse accepts none. Which file is Eclipse exactly expecting in this dialogue?
The expected spelling dictionary format is a plain text file with one word per line.
I'll point to the preference page where it says this right under the text field:
If I sound annoyed, I am. Not by the questioner, but by the people that deleted @greg-449's original answer, who seemed to have little activity in any of the tags on this question, and by SO itself for having them review it.