generate pem File for EC2 Instance at Amazon

for our productive systems we are running EC2 Instances at Amazon. The colleague responsible (say call him joe) for that has left the company. Now we want to have ssh - access to our instance. He left us both his private and his public key file

id_dsa id_dsa.pub

In the backend we can see, that the key "joe" is assigned to the runnning instance and that the ssh - access is done by the following command:

ssh -i "joe.pem" e2-user@11.22.11.123

How can we generate a .pem -file ? or more generally spoken, what needs to be done in order to get ssh -access ? Do we need Certificates from Amazon ? If so, like how ? starting help woub be great! Any hints, infos, how to's are welcome... Of course I visited the Amazon site but it was a little bit confusing. So I couldn't find the right how to.

Thanks in advance for your help!

Cheers, Ansgar