Why is the "Load putty key" field in TortoiseGit disabled?

In TortoiseGit settings go to Network. In field SSH Client, specify the path to TortoiseGitPLink

e.g.:

C:\Program Files\TortoiseGit\bin\TortoiseGitPLink.exe

Setting correct path to TortoiseGitPlink.exe for SSH client: in Settings->Network should fix the problem. Something like:

C:\Program Files\TortoiseGit\bin\TortoiseGitPlink.exe

BUT, in some cases the new path is not saved for some reason!

As mentioned in the comments, sometimes (in my case TortoiseGit 2.5.0.0) the setting was not being applied. Clicking on OK or Apply was not producing any error, but when closing and opening the Settings again, the old path was there.

In order to workaround this, check also Enable Proxy Server in the same Network menu (do not provide any proxy settings). After this the window will close on OK press, and when reopened, Enable Proxy Server will be unchecked, but the new path will be saved.