Sorry. The key exchange stuff is a place where I'll admit the
documentation is very much lacking. While iSSH should be able to take
either a putty private key file or an OpenSSH format private key file,
you may find you have more success with an OpenSSH private key file
instead. If you can, grab the key export as an OpenSSH key file and
cut and paste from the line that begins "-----BEGIN" to the line that
begins "----END". Hopefully this will work better for you.
Dean