I have created a page with hook menu with url product/checkout/%/% where % will be userid and product id. How can I add a page tpl for this url ?
In your page callback, you would have to set
'#theme' on a renderable array.
You can use an existing theme function, or create your own using hook_theme.
If creating your own and you prefer to use a
tpl, you can set the
template option in
hook_theme and bypass having to create a theme callback function.