You might have tried adding a key with:

ssh-add .ssh/id_rsa

only to get this error:

ssh-add .ssh/id_rsa
Could not open a connection to your authentication agent.[/code]
If this happens, it is because your ssh agent is not running in the terminal.

Run this:
exec ssh-agent bash

which will cause it to resource your .bashrc files and start the agent.

Now try readding the key again:

ssh-add .ssh/id_rsa
Identity added: .ssh/id_rsa (.ssh/id_rsa)