If the resource is a string resource (i.e., it is declared as <string ... in the xml), you should refer to it inside the Rez.Strings module. Failing to do so will result in a Symbol Not Found error.
As an example, if you have the following in one of your xml files...
<string id="AppName">Tic-Tac-Toe</string>
... and you wanted to access that string in your code, you'd write this...
var appName = Ui.loadResource(Rez.Strings.AppName);
The name of the resource file is not important, but the type of the resource is. The submodules of Rez are Layouts, Menus, Strings, Properties and Settings.