Solution
OPC UA Methods are functions that you can find in different clients (such as OPC Free UA Expert) and are useful since they allows the client to write to,read or browse variables (a tag) in a connected server easily.
The NI OPC UA Toolkit does not currently have the capabilities to implement these methods. In order to achieve a similar functionality is (from the Server) set the Variables as Read/Write in order to allow the client to access the value of that variable and create your own method using the OPC UA Toolkit API's.
An example of this can be found in Related Links.