It's not that simple, but it can be done.
You need to add the OCX to a form in VBA, then add your business logic - but this is advanced stuff, not for beginners.

What is the logic behind enabling/disabling the field, and which field?

Thanks for the reply. Are there some documents about the ActiveX files of particular forms? I am an experienced developer, but new to Sage 300. Is there a training course for 300 advanced customization?

Thanks for the reply. For some customers, we would like to turn on and off "Create Invoice" check box.

No
No

The quickest way to get up to speed on this stuff is to work from examples. Another consideration: Creating macros to augment logic works just fine, but without jumping through hoops, the user experience with macro-based screens is limited and can be frustrating. Experience has shown that these types of customizations are best done using VB6. Yes - that's right, I said VB6.

All that to offer some assistance. If you indicate the following, I'll create a sample project to get you over the basics.
1. Which field do you want to interact with?
2. Which version of Sage 300?
3. Do you have a VB6 dev environment available?

Thanks for the reply. I do have VB6 environment. Visual Studio 6 and 2012 are the two main developer tools I use. A sample project will be great!. I really appreciate your help!

Sorry, I forgot to mention Sage 300 2012 is the version we are using

Here's a link to a sample project that should get you up and running. Let me know if you have any questions.

www.dropbox.com/.../61_Shipment_Sample.zip

Thank you very much for the sample code. Have a nice day!