If you're not comfortable with configuring the PIX 515 via console, you should do so with the web GUI built into the the PIX OS. To set that up, console into the PIX. Then from config t, issue these commands:
http server enable
http <IP of the workstation to allow GUI access> <netmask> <interface name>
You need to set up ACL rules to allow sessions through the firewall. Personally, I would do a client to PIX VPN connection to secure the traffic. If that isn't possible, I would do as suggested by Zakkas and use SSH and not Telnet. All Telnet traffic is in clear text.