I wanted to change interpreter to IronPython in Visual Studio 2010 with Python Tools for Visual Studio. Last time I hadn't problems with this I got CPython and IronPython on list but now IronPython magically disappeared. After this I tried to restore it and done this:

I reinstalled PTVS, this still didn't helped
I reinstalled IronPython in system and still nothing

So I thought to add this entry manual but I don't know what should I enter there. I can still create project in IronPython and IntelliSense properly suggest me assembles.
Could you show me how to force PTVS to rescan to look for IronPython or what should I enter to add this entry manually.
You probably are running the latest version of IronPython and re-installed PTVS.  PTVS currently installs IronPython support conditionally on whether or not it detects that IronPython is installed.  Something changed in the latest version of IronPython and the PTVS installer fails to detect that IronPython is installed.  
The good news is you can manually select to install IronPython support.  First un-install, then start the installer and on the very first screen there's an Advanced button.  Click on that and when the feature tree comes up change IronPython support to be installed.
We've changed this for 1.5 so we'll always install IronPython support by default, so in the future this won't be an issue.
PTVS doesn't detect IronPython installation