Rovio has pulled out of their partnership with Roku and there is no method of getting back if you want to re-install. Here is the response I got from support.
Roku says contact Rovio and Rovio says talk to Roku.
Hello there,Despite what Roku support and Rovio say, factory reset works!!
and thanks for contacting us!
It seems that Roku has given you incorrect information. Unfortunately we have finished the distribution of our games on Roku platform on the 31st of August 2014. Thus, there is unfortunately no longer a way to re-install the game on your Roku.
I apologize for the inconvenience. With regards to the game you had paid for, I would suggest you contacting Roku directly to ask for a possible refund.