Can you attach the ipt file here?
Is this Inventor 11 or really 2011?
Are you in an active sketch?
What happens if you double click on the dimension?
Read over your postings so far-- do you think that you have given us enough information to understand the problem? There are several different environments within Inventor where dimensions may be entered, so you will need to give much more detail: pictures and/or files, and describe step by step what you are doing. None of us here can see your monitor or read your mind...
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