MarketPress Checkout button throws 404 error...(?)

Hi All,
Have installed Marketpress 2.4.2, GridMarket 1.4.8, added products, theoretically fired up paypal... but during testing :added products to the Shopping Cart, but upon clicking the CHECKOUT button, it throws a 404 error...

I do have it installed in a subdomain ... will this mess it up?
Also, Paypal never asked for the url (?) <>
Any help would be appreciated.
As always THANKS!