You must install the Wind River Workbench and the AdaCore compilation toolset, in that order, before installing the GNATbench plug-in for Workbench.
In all probability, you must remove or completely disable any other Ada development plug-in that includes the functionality that GNATbench provides. In particular, any plug-in that defines a project builder for Ada source code will likely prevent correct operation of GNATbench, and vice versa.
To verify that GNATbench has been installed into Workbench, use the Workbench Help menu. First, select Help and then "About Wind River Workbench". Then press the "Plug-in Details" button at the lower left of the resulting dialog box. Another pop-up dialog box will appear, showing all the installed plug-ins. You should see a number of plug-ins with "AdaCore" shown in the Provider column. If so, installation is likely fine. If not, installation has not occurred or did not complete successfully.
The list of plug-ins will look something (but not exactly) like the figure below.