I unchecked the "Allow user to administer this computer", Pls i want to tick the check box.

I transferred my documents, pictures and music from my HP PC to my new macbook pro using migration assistant. I realized I couldn't unlock the user account where it kept my transferred documents, pictures and music. I clicked the lock by the left side of the Users&Group to make changes. I was able to put in my user account password. I then unchecked the check box "Allow user to administer this computer". Now, I can't make changes like deleting files, installing new programs, etc. because it is prompting me to enter username and password. I enetered the name and password but it is not unlocking it. Please what can I do?...Pls help me. ...I want to check the box "Allow user to administer this computer", I want to tick the box but it is grayed. PLS help.