I can see the path to the Vivado install in my Matlab path.

Also, I can right-click whitespace in a new model and add Sysgen blocks with the 'Xilinx Block Add...' dialog.

What is the cause of this issue?

To work around the issue, follow these steps:

1) Make sure to close down Sysgen/Matlab

2) Copy the attached script to C:\Xilinx\Vivado\2014.4\scripts\sysgen\matlab

3) Launch Sysgen as normal from the Start Menu/desktop shortcut.

4) In the Matlab console, navigate to C:\Xilinx\Vivado\2014.4\scripts\sysgen\matlab

5) Execute the script that you just copied*

6) Close Sysgen/Matlab

7) Launch Sysgen as normal from the start menu/desktop shortcut

8) Launch Simulink

9) From the Simulink Library Browser, select 'View' -> 'Refresh Tree View'

* You may see some warnings while this script is running. 

This is normal and they can be ignored.