Hi Kieran
There are two solutions here :
1. You install and publish DMS cart module (mod_dms_cart.zip in the package). Users will see list of items added to cart and can click on checkout button to go to checkout page.
2. You can create a menu item to link to checkout view of DMS. In this case, you will need to get the file below, unzip it, get the file and upload to components/com_dms/views/checkout/tmpl folder. After that, you will be able to create the menu item to link to checkout page.
Please let me know if you need more help
Tuan