Your COBRA premiums can be entered into medical expenses if you are entering medical expenses as an itemized deduction. Go to Federal>Deductions and Credits>Medical> Medical Expenses.
The medical expense deduction has to meet a rather large threshold before it can affect your return. The amount of medical expenses that will count toward itemization for someone under 65 is the amount that is OVER 10% of your adjusted gross income; for someone 65 or older it would be the amount over 7.5% of your AGI.
You should only enter the amount that you paid in 2016—do not include any amounts that were covered by insurance or that are still outstanding. Of course, your medical expenses plus your other itemized deductions still have to exceed your standard deduction before you will see a difference in your tax due or refund.