1_ If you have a Static Public IP Address, you just have to configure your Router to redirect all the (External) incoming calls on Port 8069 to the Internal IP Address of your OpenERP Server on Port 8069.

For the majority of routers, this operation, is more commonly called "Creating a Virtual Server"

I explain, let suppose that your Public IP Address ( External IP Address) is 91.121.143.148, and that the Private IP Address (Internal IP Address) of your OpenERP Server is 192.168.1.111 configured on Port 8069.

If i am an external user, and want to connect to your OpenERP Server, i will fill 91.121.143.148:8069 in my browser address bar to connect to your Server, once this request reaches your router, this one will redirect it to 192.168.1.111:8069 (If it's configured correctly, as stated above)

2_ If your Public IP Address is Dynamic, you have to use some services (there is a free services) from NOIP or DYNDNS..., to reach your OpenERP Server from the outside, those services will provide you with a Virtual Public Domain Name, like yourcompany.redirectme.net.

All you have to do to access OpenERP Server, is to fill in your browser Address bar yourcompany.redirectme.net:8069

If you find this explanation a little bit confusing, give the Model and the Brand of your ADSL Modem Router, i will try to illustrate this explanation.