How do you change the product key for SharePoint Server 2010 after install?
To convert a license type, you must be a member of the Farm Administrators SharePoint group on the computer that is running Central Administration.
On the Central Administration Web site, in the Upgrade and Migration section, click Convert farm license type.
On the Convert License Type page, in the Enter the Product Key box, type the new product key and then click OK.
Please try the below link. It solved my problem(just as your problem). After I converted the installation type to enterprise(as documented in the below site) I could able to install the Project Server 2010 successfully. http://technet.microsoft.com/en-us/library/cc261946.aspx
And what could be the reason for NOT having enabled the Product Key Box?
As far as I remember on install I introduced my key for "SharePoint Server 2010 For Internet Sites Enterprise", but when I try to install Project Server 2010 it keeps telling me that SharePoint Server is NOT installed. I tried to check that in fact the Enterprise version was activated... but not only I can't type the key but also in the section 'Current license' is empty.