The Janome is a low shank machine and it looks as though your attachment is for a high shank. My Janome book has several walking foot attachments that are marked "excluding MC6500P" so check the Janome website for the correct attachment product number. My walking foot is specifically labelled for low shank and is part no. 214-874-024 but I don't know if this number would be relevant to you as it is about 10 years old and purchased in Australia.
I have never had any issues getting my walking foot on, it is a little bit of a fiddle, but easily done.