1. You must have more than one instrument in your score to use multi-measure rests.
2. You must first extract a part. To do this, click on the gearwheel and go to View>Dynamic Parts... Choose a part to extract.
3. Click on the gearwheel and go to Parts Settings.
4. Click on the "More" tab at the top of the window and choose a numeric value for Multi-measure rests. As an example, if you choose the number 2, anything exceeding 2 measures of rest will be converted to a multi-measure rest.