Add refurbished ESI 60D to IP900 help

[ShawnBrenner]

I am a Network/PC administrator. I need help installing a refurbished ESI 60D on a IP900. I have available licenses on my IP 900. It is a segregated network without a vlan. I tried connecting the 60D, entered the installers password, entered the next extension number... But it says "extension was invalid". Do i have to add the extension first? If so how? Sorry for the noob question, this is my first post. Anyone have installers guide? I have IP 900 & phone user and admin manuals.
Thank you

 

[ShawnBrenner]

Just for clarification: I tried connecting the Phone (ESI 60 ABP 10/100 IP), entered the installers password (on the phone), entered the next extension number (on the phone) ... But it says "extension was invalid". How do I to add the extension first (to the phone system )?