Prepare tab in file drop down menu is greyed out

Hi there

I am currently using the creo parametric 3.0 educational edition.

I would like to check / change the measurement units. However, the "prepare" tab in the file drop down is greyed out. 

I've use the magnifying glass search section on the top right corner and find model properties, and it is also greyed out.

Then I tried the "dimension" function in the "Tools" header tab, that option is also greyed out.

So does anyone know how I can solve this issue or what other ways I can change/check the units? Many thanks