You do not report the K-1 if the investment is held in a retirement account.
Tax rules require tax-advantaged accounts like IRAs to pay income tax on what is called "unrelated business income." An IRA holds investments and partnership income may not be investment income, resulting in UBI from your partnership investments. The amount of unrelated business income from your IRA's partnership investments will be listed as such in Box 20 of the K-1 form. Only the amounts of unrelated business income are important concerning taxes and your IRA account.
If your IRA earns more than $1,000 in unrelated business income as reported on the K-1s you received, the IRA must pay unrelated business income tax – UBIT – on the amount above the $1,000 cutoff. Not all of the distributions you receive from MLP investments will be UBI, and many partnership investments will report little or no UBI on the K-1s they send out. If the K-1s you received total to less than $1,000 in UBI, there is nothing you need to do. With more than $1,000 in UBI you need to contact your IRA custodian.
Your IRA and not you is responsible for paying any UBIT. The custodian of your IRA will complete and file a tax return in the name of your IRA account. The custodian will charge a fee for this service and any taxes due will come out of the IRA value.