ActionScript 3.0 :: Adding A Web Url Link To An Existing File: 1046: Type Was Not Found Or Was Not A Compile-time Constant?

We're trying to add a simple link to an existing Flash file. There looks to be at least 7 separate .as files and a separate swf that loads the main swf which all seems overly complex for what is essentially a page with six buttons on it.However, we need to add a URL to some of the text. So we converted the text to a button, added an instance name of <ssbpurchasetickets_btn> then added the following Actionscript into the actions layer on the frame the text/button appears:
 
ssbpurchasetickets_btn.addEventListener(MouseEvent.CLICK, ssbButtonPurchase);
 function ssbButtonPurchase(event:MouseEvent):void
{[code]......
 
When we publish the file we get the following error which seems to cascade into a whole bunch more errors:Description: 1046: Type was not found or was not a compile-time constant: MouseEvent.Source: function ssbButtonPurchase(event:MouseEvent):void

We added "ssb" onto the button and functions to ensure there were no conflicts but the same thing occurred. If we copy the button into a new file everything works so it must be conflicting with something in the main files.