If the button is for an active spell (i.e. you should be able to click the button) make sure the normal (nondisabled) button has the parent filepath:
ReplaceableTextures/CommandButtons/
and the filename is prefixed with BTN, so for example:
ReplaceableTextures/CommandButtons/BTNSomeButton.blp
And the disabled button has the parent filepath:
ReplaceableTextures/CommandButtonsDisabled/
and is prefixed with DISBTN but otherwise has the same filename as the nondisabled button:
ReplaceableTextures/CommandButtons/DISBTNSomeButton.blp
If the buttons are for a passive ability, then it's the same deal only you prefix the filenames with PASBTN and DISPASBTN (I know I read somewhere that DISPASBTN doesn't work but that is wrong, if you try using PASBTN and just plain DISBTN the disabled button won't register).