Rests are generated automatically by QuickScore Elite, so normally you do not have occasion to enter rests yourself. If you don't like the way QuickScore Elite has chosen rests in a given bar, you can choose not to display the default rests and enter them yourself. To do this, select the Bar dialog under the Display menu and set Display Rests to No.
Now you can enter the rests yourself as symbols from the Notes symbol palette. Click on the symbols button twice in the object type palette and then click on the Notes button in the symbols palette. Choose the rest you want to enter. Select the pencil tool in the toolbar and enter the rest where you want it.