It could be a typo: H is very close to M on a UK or US keyboard.
IHG = IMG, maybe, just my 2 cents.
He might be talking about an xpi file.
If we are talking about an xpi file, this is what you do:
in palemoon Tools > addons, in the addons page, top right, small pic of a gear, click on the small right down arrow, very close, select install add-om from file, browse to the xpi file you saved.
He might also be talking about an xml file, which will be another story... let's wait for his answer.
If nothing is working, tell us in more details about your problem.