Where can I find an Active X control to connect PLCs (such as AB
SLC500 or Micrologix) to Visual Basic 6.0 protrams?

Here is a short list of places that have ActiveX controls for VBX. The
ones at INGEAR mention the Allen-Bradley and MicroLogix by name.
Hopefully you will find something from the companies below that will
do the job for you at a reasonable price.

INGEAR OPC ( http://www.ingear.cimquest.com/ )
http://www.ingear.cimquest.com/allen-br.htm
http://www.ingear.cimquest.com/allen-br1.htm

ConsoliTech Corp ( http://www.consolitech.com/ )
http://www.consolitech.com/PLCOCX.htm

Automated Solutions
http://www.automatedsolutions.com/

Parijat Controlware, Inc. ( http://www.parijat.com/ )
http://www.parijat.com/32bit_comm_drivers.htm

Software Toolbox ( http://www.softwaretoolbox.com/ )
http://www.softwaretoolbox.com/store/item_pages/itempage_659.asp

Anyway, that should get you started. There were more hits from the
search I did, but why should I pad the answer out, when you can find
them yourself with a simple click on the Search Strategy Link below?
:)

googlebrain-ga

Search Strategy:

PLC "ActiveX"
://www.google.com/search?sourceid=navclient&q=PLC+%22ActiveX%22

Thanks for the help. This question is very specific to a particular
field of control engineering, so I didn't know if I would get a
specific answer at general site like this.

Allen-Bradley Ethernet Compatible ActiveX Toolkit 
 
Product Summary:  
 
A-B Ethernet ActiveX Toolkit provides high-performance connectivity to
A-B ControlLogix, PLC5, SLC500, and MicroLogix PLCs via A-B Ethernet
networks.
 
Use Visual Basic, Visual C++, Excel, Delphi, C++ Builder, or other
development packages to create HMI/SCADA applications using our
runtime-free ActiveX Controls.
 
 
http://www.automatedsolutions.com/products/abtcpip.asp 
 
A little pricey but it will do the job.