You can use the following free openssl tool to convert a DKIM private
key from pfx format to PEM:
Once you have installed the program and opened it, simply issue the
openssl pkcs12 -in c:certsyourcert.pfx -out c:certsyourcert.pem
Next, open the PEM file in a text editor (such as NotePad or
EditPlus) and ignore all but the last certificate section.
You can then copy and paste everything from the begin tag to the end
tag (including the tags themselves) into the private key text box.