Ever since I upgraded to the latest version of Texmaker the spellchecker started to show the following error Can’t open the dictionary. So after struggling a bit with it I realized that the dictionary file for English US (en_US.dic) I had obtained from the openoffice repositories as recommended by the texmaker documentation did not have the affixes file (en_US.aff). So I read a bit about it a found out the following:
The .dic file is a list of words along with a group of letters which refer to the affixes found in the .aff file. This saves space because instead of having include all forms of a word, like jump (jumping, jumps, jumped), the .dic file will include the word once and the references to the affixes in the .aff file allow the construction of all the other forms.
Solution: Download an up-to-date spelling dictionary that includes this aff file. You can get one from wordlist.sourceforge.net. Unzip, put it wherever you want, and then head to the preferences->editor in texmaker and point to where you have your .dic file. It should work.