As a new seller there are several restrictions on your account.
The restriction of CDN$ only is because you are using the simplified Sell Your Item Form.
Go here:
http://cgi5.ebay.ca/ws/eBayISAPI.dll?SellHub3
Which is the more complex and detailed form.
The choice of US or Canadian currency is on the fourth page in the
Categories where your listing will appear
section.
Other things you may notice are that the number and value of items you can list are restricted and that your customers' cleared payments will not be released for 21 days.