OEM partners like Dell usually provide customers with a PAC (Partner Activation Code), rather than a license key.
You should have received either an Email, or find instructions on the site on which you found that PAC (likely Dell's Digital Locker, for how to activate that code in the VMware customer portal.
A quick search on the Internet shows http://www.vmware.com/vmwarestore/newstore/oem_login.jsp?Name=DELL-AC as the activation URL.
André