I am on Medicare but not yet collecting Social Security. Can I deduct the cost of Medicare and where do I enter this information?

Medicare premiums are a medical expense if you itemize. You can only deduct an amount of total medical expenses greater than 7.5 percent of your AGI. 

Where exactly do I enter the Medicare premium amount?

In the Deductions & Credits page..

There is a menu item for Medical Expenses

Start in there, and there are entry areas for both un-reimbursed out-of-pocket medical, eye, dental expenses, and any medical Premiums you may have paid.

You should probably go thru the interview for that section twice, to see how the software breaks out all the different items, so you can see where the best place is to enter those expenses during the interview.