there is no such option as "Portal Access Management" in Employee options...

can you specify me the location...

I think he means you to go HR -> Employee ->edit -> Related User -> from dropdown select option create and edit and it will create you user connected to that employee but not sure if that is direct enough for you based on your question I would assume you want something more direct.

For creating Employee user, I have to create a customer linked to the employee who is an user to the OpenERP instance... Is my understanding correct?

No it creates just ordinary user but all users are handled as OpenERP Partners so you will see it in almost all dropdown menus because they are showing partners not only customers.

thanks, yeah i got your point of view