If you are using the defaults for both IIS and Tomcat, they should actually work side-by-side without a hitch.
IIS would be accessible through port 80 (i.e. through http://localhost/ or http://localhost:80/ - when accessing through a browser running on the same machine), whereas Tomcat would be accessible through port 8080 (i.e. through http://localhost:8080/).
If you are using the defaults and tomcat is not accessible, have a look at the tomcat logs (in the logs subdirectory of the tomcat installation), and report any error which may appear
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