There are a number of ways you can do this with different degrees of difficultly, flexibility, and re-usability.
The easiest is to directly reference the instance on the MainForm. For instance:
Dim MyValue as string
MyValue=MainForm.ModbusRTUCom1.Read("40001")
This method is the simplest, but is the most encouraged because you are creating dependencies that can easily break the application. For instance, what if the driver is removed from the MainForm or a different driver added.
Another method is to create an instance of the driver within your class. For example:
Dim MyDriverInstance as ModbusRTUCom
MyDriverInstance=New ModbusRTUCom
MyDriverInstance.PortName="COM1"
MyDriverInstance.BaudRate=19200
etc.....
A third method is to create a ComComponent property in your class and have the code that instantiates your class set the ComComponent property. You would then use the property to access the driver.
If you are creating a class with shared methods, then you can simply pass the driver instance as a parameter.
Public Shared Function MyFunction(ByVal driver as IComComponent)