Make absolutely certain that your servlet container is working correctly. If you cannot connect to the demonstration examples that come with your container, then webMathematica is unlikely to work. Furthermore, Wolfram Research will not be able to give more than minimal assistance until your servlet container is working.
If your servlet container works but webMathematica does not, make sure you are using a correct URL. These are case sensitive, so make sure you use capital letters in the same places as the documentation describes. For example, http://localhost:8080/webMathematica/Examples/Hello.jsp is the appropriate usage. Make sure that this URL is consistent with the way you connect to your servlet container. If you need to specify a port number for your servlet container, you will need to use this for webMathematica. For example, the default settings for direct access for Tomcat is port 8080. A URL for a different servlet container might be different. The URLs in this document are all specified for Tomcat.
If the initial page that is returned to your browser in response to a webMathematica URL, such as http://localhost:8080/webMathematica/Examples/Hello.jsp, contains some other indication of an error, study it carefully. Some typical problems include configuration errors, such as failure to locate a configuration file or failure to launch Mathematica. Studying the initial page and rereading the installation instructions or looking at the specific problems listed later in this section may help to resolve the problem.
The kernel monitor contains information on the configuration of the webMathematica site and also can print out information if your server is misconfigured. You should be able to find the monitor via http://localhost:8080/webMathematica/Resources/Tools/KernelMonitor.jsp. (You may have some different URL for accessing your server.) The monitor is described in more detail in a previous section.
The logging system is a good place to start to search for information on problems.
Run Mathematica in the same way that it is run from the servlet container. For example, under Unix, the servlet container often runs as tomcat, so this should be used to run Mathematica. Do this for both the Mathematica kernel and front end.
Running Mathematica like this will help to identify problems that may prevent the web system from launching Mathematica. One source of problems is caused by a failure to find a license to run Mathematica. You can resolve this by making sure that the valid Mathematica license is present in the Mathematica layout, probably by placing a mathpass file into $InstallationDirectory/Configuration/Licensing.