If you have already linked your MTurk account to TurkPrime, add the funds to MTurk and it will be automatically linked. It should then show your balance on the top of the TurkPrime website under ‘MTurk Balance’. If you haven't linked your accounts, or would like to add funds without using an MTurk account, you can click on where it says ‘Lab Balance’ on the top of the page. You can also enter your account and on the drop down list you will see options to fund your lab account or fund your Mechanical Turk account (which will just re-direct you to the MTurk funding page).
Please note that it may take a few minutes for MTurk to complete processing your funding before it becomes available and visible on your account.