When I am installing Vivado 2015.1 or 2015.2, I get to the "Select Destination Directory" page but the "Select the installation directory" entry box is grayed out.
Under the Installation location(s) there is an orange info icon next to DocNav, or both Vivado and DocNav.
The note at the bottom of the page related to the info icon states the following:
If the value in the location directory field is correct, you can use the current value and continue with the installation.
The problem is caused by the way in which the installer evaluates the DocNav directory when a version of DocNav has already been installed on the computer (for example, from 2014.4 or earlier).
To enable the install directory location field, do the following:
DocNav does not need to be reinstalled with each version of Vivado.
However, if you wish to install DocNav over the currently installed version, use the steps above to enable the install location.
Then after you enter the desired install directory, go back to the Tools selection page and select DocNav again.
When you select Next, the directory location will again be disabled but will retain the desired directory value.
This issue is fixed in Vivado 2015.3.