You are not supposed to create "Password" attribute yourself - the system will do it for you.
There is a business object called "RegularUser" that represents a user of the system. You can use this object. Alternatively you can create your own object to represent a user. If you do this make sure that you add this object to the SystemUsers business object group. When you do this Aware IM will automatically add the "Password" attribute (and a few others).
For more details please see the "How to set up system users" section in the "How to" document.