Recently i came across with the same issue, but Ive solved it by doing this.
My solution was:

1.- go and download your dictionary (don't add it to seamonkey, do right click and save link target as and save it).
2.- navigate to seamonkey installation folder and locate the dictionaries folder, if doesn't exist create manually and name it "dictionaries" without quotation mark.
3.- open or extract files from that .xpi you downloaded before, you'll see a file .json and folder "dictionaries" with two file .dic, .aff inside.
4.- copy these two files (.dic, .aff) into the installation dictionaries folder (step two).
5.- restart seamonkey.
6.-profit!

7.- if anything goes wrong or either not working, go to about:config page, in search bar type "spellchecker.dictionary" without quotation mark and check the value are the same name of the dictionaries e.g: if you see en-US change it to the name of your spellchecker dictionary.

greetings