I live in wisconsin and want to attach a Rent cert9f9cate to my efiled returns -- how can I do this.

If you have not already e-filed your returns yet, TurboTax will ask you to attach it when you go through the Wisconsin Homestead Credit Claim questions.   

If you already e-filed your return and need to add the homestead credit, you will need to file an amended return.  See How do I file an amended state return?