You're referring to the Lifetime Learning Credit (LLC). It's not limited to the first four years of post-secondary education like its larger cousin, The American Opportunity Credit, so you can use it for grad school.
The LLC is a non-refundable credit equal to 20% of qualified education expenses up to $10,000, so the maximum credit is worth $2000. There's no workload requirement, so you can be full-time or part-time, and it counts for degree or non-degree courses, as well as courses for job-skill improvement. And there's no limit on the number of years you can claim it.
If you didn't claim it in 2014 & 2015, then you can file amended returns for those years to add the education expenses. So you haven't lost out on it.
Here's a link to IRS Publication 970 which discusses all aspects of the credit: https://www.irs.gov/pub/irs-pdf/p970.pdf.
Additionally, here's a link to the TurboTax support page for amending returns, which will get you started: https://turbotax.intuit.com/tax-tools/tax-tips/IRS-Tax-Return/Amending-Your-Income-Tax-Return/INF120...
