how do i deauthorize my computers without access

I recently purchased a new Macbook Pro and am trying to authorize the computer through iTunes using my iTunes account. Unfortunately it tells me I already have 5 computers authorized and won't let me proceed. It also won't give me the menu option to de-authorize my existing computers.

These existing authorization are from a few different old machines to which i no longer have access.

How can I de-authorize them so that I can authorize my new rMBP ??

I am running the latest version of iTunes on 10.10.3 on a new 2015 retina Macbook Pro.

Thanks!!!

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John