Unable to use key file

linux ssh putty

You will need to convert that key using PuTTYgen (http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html) to a key usable by PuTTY.

The steps are:

Download PuTTYgen.
Load your key
Save the private key

See https://unix.stackexchange.com/questions/74545/what-difference-between-openssh-key-and-putty-key