since it is not checked in box 13. HR's words might be more important than any answer here, including mine.
If it turns out that you are indeed covered by a plan (and somehow the box is not checked mistakenly), and you did not put any money in, then, I am not sure anything you can do about this, i.e. taking deduction for your t-IRA contribution. Depending on where your income stands between 89K-109K, you may contribute a portion (of the 5000) as deductible contribution.
To tax-saving for the future, if you can not deduct all the 5000 (let's see, you can take 1000 as deductible contribution), you should consider putting the rest (5000-1000=4000)into your Roth IRA. It can not save tax now, but any gain in the futre will be tax-free.
It is a lesson, any way. I have wrtten a lot about tax saving strategy and planning here, but not many people tried to understand them. Some of them even claimed that they are useless since they did not get it. I hope your will plan in the future.
The best.
