After connecting to the LHC@home BOINC project web site, please navigate menu item: "Your account" on the left. From there, please go to "Preferences for this project" and click on "LHCathome preferences". This will bring up the screen in the clickable image above.
From here you may choose your preferred application, e.g. CMS simulation if you wish to prioritize the CMS experiment. In general we recommend to leave ticked the box: "If no work for the selected application is available, accept work from other applications." With this setting, you are sure to get tasks from CERN even if there is a temporary shortage of tasks for your favourite experiment.