A few questions....
1. Have you looked at the event viewer on the OCS server to see if the 500 error generated an application error.
2. Sometimes you can tell alot about whether the application is even installed properly by simply typing the URL into a web browser to see the response... especially with SALT apps. So can you put the URL into IE for example and at least get a response from IIS.
3. Are you using a soft phone or some other method to call the application on your OCS box?
4. Where you able to call the OCS box with the default "Welcome to Speech Server 2007" application before attempting to call the SALT application?
kstep