The next screen says "Now you may also choose a specific Computing Element where to start the rendering of the protein." A Computing Element (CE) is basically a computer or a group of computers (sometimes called a cluster) on the grid which are available to do the computing.    

To see all the CEs available, choose the drop down list labelled "Specify the CE Resource". You will see that each CE has three different entries. This represents the 'job queues' allocated for each CE; 'short', 'long' and 'infinite'. 'short' jobs are those that are expected to take no longer than a few minutes to complete, 'long' jobs are ones which may take a few hours and 'infinite' is for really long jobs. The reason for this is to enable short jobs to be run on the CE without having to wait for an infinite job to complete.

Choose the top option in the 'Select CE' drop down, 'Let the GILDA Resource Broker choose'. Once you have made your choices, click the 'Start Protein Rendering' button.