Form 3531- Where do I mail it back to?

[Minions03]

Hello, 

I received Form 3531 (Request for Signature or Missing Information to Complete Return). When they sent this to me they did not have a return address anywhere. Where am I supposed to return this form to? 

[Critter-3]

The actual Form 3531 states at the bottom of the second page:

TO REPLY, USE THE RETURN ADDRESS ON THE MAILING ENVELOPE OR REFER TO IRS.GOV AND SEARCH

"WHERE TO FILE."

[minion03]

There was NO return address on the envelope and I have looked everywhere on the IRS website for this form and cannot find it. I even tried calling them and have had no luck getting in contact with someone.