How to file tax form 5498
Form 5498 is an information return that describes contributions that you made to your IRAs during the year. The Roth IRA conversion amount is in Box 3. You don't need the 5498 to file your return; the custodian of your traditional IRA will issue a Form 1099-R showing the distribution from your traditional IRA. While you can use the info on Form 5498, you should use the 1099-R, as that is what the IRS will use.
If you made the conversion in calendar year 2016, you do not have to do anything until after the end of the year, when you receive the 1099-R from the custodian. The 1099-R will contain the information needed to compute the taxes that will need to be paid. The Roth conversion is reported on Form 8606, on lines 16–18 of Part II of that form (assuming you took no distributions).
If you made the conversion in calendar year 2015, you should have received the 1099-R from the custodian and included it on your return. If not you will have to amend your 2015 return to include it.