Facebook will allow you to add a Facebook app to a "Page" but not a "Group" and you need to be a Facebook Page Administrator to add the "buy tickets" button. To create a Facebook Page, visit https://www.facebook.com/pages/create.php
If you are a Facebook Page Administrator and you have a page (rather than a group) it may be that the 'buy tickets' button has dropped off the page. Next to the section where you have items such as 'photos', 'likes' and 'map' showing, you may have a drop down arrow with a number showing, indicating that there are more links or buttons not currently showing on the page. Click on the drop down arrow and the 'buy tickets' button should now appear. Hover over the 'buy tickets' button and a pencil icon will appear in the top right hand corner. Click on the pencil icon and you will be provided with the option to 'switch positions with'. Select the item to switch with.