A great reference for getting started with the Web UI Builder can be found here.
To allow the Web UI to monitor remote I/O, you will need to build the web service on the RT target (or you could build the web service on the host and use a different method such as shared variables to get the data to the host). To enable web services on the RT target: right click on the target, select "Properties" and then select "Web Server" in the list that opens. Ensure that the web application server is enabled, and also enable the remote panel server.
If you are using the Web UI to control anything on the cRIO, you also have the option to deploy the Web UI Builder VI to the cRIO. To do this, in the Web UI Builder, select "Build and Deploy Project" from the Project tab in the Web UI Builder and then select the "Deploy to RT Target" option.
Hope this helps!
Julianne K Applications Engineer National Instruments