Yes, the installer needs to be told where to install the essential_viewer.war because different application servers have different structures. If you are using Tomcat, then select your <TOMCAT INSTALL>/webapps folder. The installer should have some instructions about this on the relevant step of the install process.
The error message that you are seeing means that the Essential Architecture Reporting Tab cannot connect to Essential Viewer. I think we should start by making sure that you have installed Essential Viewer correctly.
First, check that the installer has deployed the file 'essential_viewer.war' to the <TOMCAT_INSTALL_FOLDER>/webapps folder. Start Tomcat and this WAR file should then be unpacked to create a folder called 'essential_viewer' in the <TOMCAT_INSTALL_FOLDER>/webapps folder, e.g.
<TOMCAT_INSTALL>\webapps\essential_viewer
If you do not have this folder in your webapps folder, then the Essential Viewer has not been installed correctly. You can either find the 'essential_viewer.war' file (e.g. in the <TOMCAT_INSTALL> folder?) and copy it to the 'webapps' folder or re-run the installer and select ONLY the Essential Viewer pack to install.
If you do have the 'essential_viewer' folder in 'webapps', then Essential Viewer has been installed correctly. In this case, ensure that Tomcat has been started and test that it is responding by opening a web browser and navigate to the Essential Viewer homepage:
http://localhost:8080/essential_viewer
You should now see the Essential Viewer homepage. If not, check again that Tomcat is running and that Essential Viewer has been installed correctly. The Essential Architecture Reporting Tab in Protege should now be able to successfully connect to the ReportService.
If you are still having trouble, check that you have the correct URL in the Essential Architecture Reporting Tab. The default URL is:
http://localhost:8080/essential_viewer/reportService
If you are still having problems, check that there is not a firewall between the server hosting Tomcat and the client running Protege.
Hopefully this should get the Reporting Tab connected to Essential Viewer. Let me know how you get on.
Jonathan