Open iTunes on your iMac.
In the lefthand column of the iTunes window that opens, under Library, click on Apps. A window will open that will show all the Apps you've downloaded. If you downloaded the App via the iPad, you will have needed to sync the iPad and the iMac previously for the App to be listed. (I think)
Click on the App you want to delete, then press the Delete key on your keyboard. A window will open asking you to confirm that you want to delete the app. Click the confirm button.