What province are you in? Which version of TurboTax are you using? Is it Online or is it the CD/Desktop edition?
I live in Ontario and I am using the free online version of Turbo Tax.
From the left side menu, select Provincial>Provincial Tax Credit Profile. Put a check next to anything on the page that you wish to claim, and click Continue. On the next page you’ll be asked questions regarding the Ontario Trillium Benefit, including the rental credit. Answer YES to whatever you qualify for and click Continue. Then you’ll be ask for you rent info.
When using TurboTax Online-
From the left side menu, select Provincial>Provincial Tax Credit Profile. Put a check next to anything on the page that you wish to claim, and click Continue. On the next page, you’ll be asked questions regarding the Ontario Trillium Benefit, including the rental credit. Answer YES to whatever you qualify for and click Continue. Then you’ll be asked for your rent information, that is when you will enter how much you paid.