Agree with above - you cannot work on a job-seeker visa so this is not worth the trouble.
There might be some confusion around the word "intern" here - sounds like your future employer only wants to hire you as a freelancer. You are not formally any sort of intern, which I believe would have certain benefits. They are probably trying to save themselves some hassle by not treating you as an employee or student, without realizing that this doesn't change your need for a proper work permit.
However, given the relatively generous treatment received by North Americans (no need for visa before arrival and all that) I would try the following strategy:
1. Arrive in Germany, immediately register your address at the local Burgeramt or whatever it's called where you're living.
2. Head directly to the Ausländerbehörde (make an appointment before you leave the US, if possible, or go at six in the morning if there's a daily queue) and apply for a six-month residence permit to do your internship. Bring, among other things, proof of health insurance (which can be US-based travel insurance if it meets the certain requirements) plus proof of sufficient savings to get yourself home if there's a problem, a copy of your return ticket, university degrees and high school diplomas or whatever, and a letter from your potential employer stating the terms under which they wish to hire you. Make a good case, and if all goes well, you'll have a stamp in your passport that morning; if not, enjoy your stay as an unpaid tourist.
If you intend to find a permanent job and stay longer, that's a bridge I'd cross when I get there. Buy a return ticket that you can cash in or easily change, but have one with you as proof of your intention to return.
Last time we were over, I did something similar and it was very easy. But it was clearly a temporary stay - my wife's sabbatical semester. I had an offer letter for a project, showed that along with my CV, asked if I might be allowed to work as a freelancer, and was immediately given an unrestricted work permit for the five months I was there. I could have worked in a bar or kebab shop if I'd wanted to.
The "freelance visa" is an option worth exploring too - intended for creative industries and language teachers and whatnot. There are requirements that you show a bit of a business plan, whereas in your situation you already have an offer.
The safest alternative might be to pursue the business visa before you leave the US - then at least you're not getting on a plane not knowing if it will be approved. I'm not familiar with it so can't offer additional advice.
I've worked on freelance contracts a couple of times and, to my mild surprise, noticed that employers don't seem to care too much about your immigration status or check very carefully or even know what the rules are. If you were working for six months then technically you should have a German tax ID for invoicing and all that, but they'd probably be fine without one (you'd bill them with a US address and SSN though your could probably still be paid locally) or they'll decide it's some sort of tax-exempt student internship thing.