Grabbers are listed on the "Grabbers" tab of the configuration screen:
To enable or disable a grabber click on the box on the left of the grabber name. To change a grabber's settings click on the grabber and then click on settings. To select the channels that the grabber should download click on the grabber and then click on channels.
To add a channel select it from the list on the left and then click the ">>" button. To remove a channel select a channel from the list on the right and click the "<<" button. To change the order of your selected channels select the channel you want to move and click on the "^" and "v" buttons. To check for new channels click on "Refresh Channels", this option is not available on all grabbers.