You have installed the VirtueMart component and can add products to the cart - but you cannot see the cart and have no link to checkout.
If so, you need to publish either the minicart in the main VirtueMart module, mod_virtuemart, or publish the VirtueMart minicart module, mod_virtuemart_cart. All accomplished through Joomla's module manager.
Similarly if you don't see a customer login/register link, this can be shown in mod_virtuemart (if published) or in mod_virtuemart_login.
If you do not see the modules in Joomla's module list then you should install them. They are in the modules directory of the VirtueMart Complete Package (from http://virtuemart.net/downloads