Hey Tim I am always glad to help a fellow programmer bitten by the bug. I will try to clarify the difference between an ActiveX control and an ActiveX DLL (basically a standard DLL except it can be used in an ActiveX compatible program).
First of all the Mitsubishi serial port master driver from the previous link is an ActiveX DLL. That is all you need to communicate with your particular plc using VB.
Second your link to the VB.Net standard was not directed to the standard (at least on my machine), but I can tell you right now it will work fine.
OK, I will try to keep this simple for beginner VB's. It will all come together when you start using VB.
With an ActiveX you just drag the control to your form in VB. From there you can set certain properties, like the baud rate and such, right from the properties list. You still have to call the methods (tell the ActiveX to talk using the properties you set) using code.
With an ActiveX DLL you don't drag the control to your form. You just include it in your VB Project and then you set all of the properties and call the methods in code. You must also declare the functions you want to use.
Now, I know It's starting to sound complicated, but trust me it is not, and the rewards are great.
The ActiveX that you purchase will have some example code for you to use. The most important thing you can do is get familiar with VB and writing code. I started and still am a PLC programmer and trust me if you can program a plc you can easily learn VB and make this project work.