As stated in Subject!
You don't.
If you would have used the search function, you would have found that this (non) issue has been discussed many times before, just not in this section but in the 'Website and Forum' section instead.
Mortal uses can only see the IP address they posted from.
They can not see other user's IP addresses (can you see mine ?).
The 'immortal' users, moderators and administrators, can see anyone's IP address, and actually need that to do their job.
So don't worry.
You're still safe.
If you don't like the moderators and administrators to see your IP address, stop posting on any forum right now.
Any and all forum software i've seen so far, works exactly like that.
How do you hide your IP address shown against each post.?
https://www.google.com/search?q=tor
Bear in mind there is evidence the network has, on occasion, been compromised by the FBI.
And the CIA, NSA, KGB, MI6, Mossad, Fox News, Julian Assange, Google ...
Does anyone still believe they can have secrets?
In the case of arduino.cc, it's probably a good thing the FBI is watching. Surely terrorists come here to learn remote sensing and detonation. The pedagogical philosophy of arduino.cc means that terrorists know how to read data sheets and use ohm's law! CIA torture evidence indicates that terrorists hate Fritzing even more than they hate infidels.
ChrisTenone:
...Fox News...
Naw. They can't even figure out how to verify a resume...
that terrorists hate Fritzing even more than they hate infidels.
I hate Fritzing but I am not a terrorist 
68tjs:
I hate Fritzing but I am not a terrorist
They get that way from reading this site. (after all, they are probably easily influenced.)
it probably isn't hard for someone to hack it so they appear as a moderator or at least to the part of code that determines whether to show the IP address.
turtle_god:
it probably isn't hard for someone to hack it so they appear as a moderator
Your mission, should you chose to accept it, is to do just that.
Of course, then you'll be a moderator, and you'll have to deal with all that that entails.
Delta_G:
Last time I came here through tor it wouldn't let me log in.
That may be a result of how Arduino handles single sign-on. They seem to use a combination of cookies and redirects. Maybe Tor has a side-effect of crushing all that hand waving.
What I remember is that the encryption is not compromised. (Sans quantum computer I don't think it can be.) The anonymity is. (Albeit that is the point of Tor.) For someone in the U.S. who is not engaged in criminal activity it is a distinction without a difference. For a special forces operative it can mean the difference between an intimate talk with a spouse and fleeing from the enemy. For a dissident writing about the misdeeds of certain governments it can mean the difference between life and death.