No idea about the Can/US dollar question (my store is on .com and I don't have that problem).
You can add new categories or sub-categories by going to 'Seller, Manage Store' - at the bottom right of your store front page - and selecting 'Store Categories' from the menu on the left. Click 'Add category' to set up a brand new level one category; click on an existing category, then 'Add category' to set up a sub-category.
Ann