This is the wrong forum for this question, but whatever.
The short answer is, you don't. The description is fairly meaningless, and is for display purposes only. Once you add the user and set its password, it then appears in the user list. To give it "full access", which essentially means local administrator privileges, you then either right-click on the created user in the user list, select Properties, go to the "Member Of" tab, and add the "Administrators" group to its group list, or go to the Groups option in lusrmgr, right-click on Administrators, select Properties, go to the "Members" tab and add the user. The two methods accomplish the same thing.
If the system won't let you add a user at all, then you're stuck. You could reinstall Windows, or try to enable and reset the password of the Administrator account by using a Linux boot CD (or flash drive) with utilities designed for that purpose, but the details of that are beyond the scope of this question.