If you're talking about the previous year's agi it should have been retrieved when TurboTax pulled in the previous year's tax information for the current year's form. Look at the next page and see if it is already there. Mine was.
Otherwise, if you saved.pdf files for previous tax returns you should be able to look at the.pdf file on your computer and find the AGI that way.
In order to look at the.pdf file, you may need to install the appropriate Adobe reader program. These are free from Adobe.
Beyond that I cannot help you.