You don't need to do anything specific to view the file service_proxy_access.log, which is located in /var/log/apache2/
How to find it if you don't use terminal:
1. Click on Finder
2. Go to the Go menu
3. Click on Go to folder...
4. Type /var/log/apache2/ in the text field and click on the Go button
5. Left click on the file service_proxy_access.log
6. Click on Open with...
7. Select TextEdit (or better TextWrangler if you have it installed on your machine)
Now, I found it better to edit the conf files so they could be shown using the Server app.
However, if you have updated to Server 5.0.15 you should read Access Log reports all visitors as from 127.0.0.1 or - - which deals with the new problems the update introduced.