Entered CPP contributions from box 16 on T4 into the appropriate box (16) in Turbo Tax, but CRA says it should also be in line 308 of the return. How do I see line 308

You can access a detailed summary under the "Review" tab > "Bottom Line (tax summaries)" > in the lower left hand corner (paper + magnifying glass) click on "Detailed Tax Summary"

Once the cost of Turbo Tax has been paid for;

You can also access a full PDF copy of your tax return in the Bottom Line screen under the paper icon and select "Review T1 General"

It may also be advisable to double check your slip entry as missing typing a number is a common mistake.