First try clicking in the box first and then try to enter the number.

If you are still having trouble enter the amount, you can enter your property taxes by following these steps:

1. Click on Federal > Deductions & Credits 
2. In the Your Home section, start by clicking on the Start/Revisit box next to Property Taxes.  
3. Enter your taxes on the next screen, Enter the Property Taxes You Paid.  
   

Just don't enter your property taxes in both places!

I tried the process you described. It look me round about to the exact same form. The Property Tax field on the form is still read only! 

I could click 'Show Form' in the lower right of the screen, 
On the 'Tax and Interest Deduciton Worksheet'
Section 2 
State and Local Real Estate Taxes

I was able to enter both my Property tax and HOA dues.