Different sites for MBUTTON
ftp://ftp.winsite.com/pub/pc/winnt/miscutil/mbutton.zip
ftp://ftp.the.net/mirrors/ftp.winsite.com/pc/winnt/miscutil/mbutton.zip
ftp://ftp.tu-chemnitz.de/pub/cica-winnt/miscutil/mbutton.zip
ftp://ftp.tas.gov.au/pc/winsite/winnt/miscutil/mbutton.zip

This a part of the readme:
=======================
MButton is an EXE and DLL that hooks into the middle button message
and generates a double click instead. It will work with both Windows NT
(i386) and Windows 95. However, it is aimed for users of Windows NT who
have a 3 button mouse and wish to have middle button functionality.

To have Windows NT recognise the middle button mouse you need to set the
registry correctly. Set the following value to 0x03.

For a bus mouse:

HKEY_LOCAL_MACHINE\\SYSTEM\\CURRENTCONTROLSET\\SERVICES\\i8042prt\\parameters\\NumberOfButtons

For a Serial Mouse:

HKEY_LOCAL_MACHINE\\SYSTEM\\CURRENTCONTROLSET\\SERVICES\\Sermouse\\parameters\\NumberOfButtons

Or something similar for other type of mice.
[...]
===============================
You can e-mail the author and it is freeware.

Maybe for your Problem you do not need the MBUTTON program and changing the registry is enough.

Hope this answer is what you expected.

Cer    
What is DLL? Where do you find and how do you set registry?

I am not needing the middle button to replace the double click, will software that I am hoping to use be able to use it for a more specialised function?

I was able to find the sites you suggested for MBUTTON but the Microsoft Internet Explorer that I am using seemed unable to open them. Is there any other way I could obtain the software?

Do you think my PRIMAX mouse would work if I did what you suggested?

DLL = Dynamic Link Library. A code library that is used by EXE files. I could explain more, but it would help you.

The suggested MBUTTON is ONLY to get the doubleclick on the middle mouse button, if you don't want this, there is no need to load the file.

Internet Explorer MUST be able to open the site. i guess you entered the path wrong. You must copy the complete line to your browser. Normaly your browser shows something like:

   http://server/dir/page.html

you must not use http, you must use ftp. It looks like:

   ftp://server/dir/file.zip

the MBUTTON.ZIP file must be expanded using PKUNZIP or WINZIP, it contains the EXE, DLL and instructions.

I suggest you first edit the registry as suggested. this tells NT that you have a 3-Button mouse and NT will generate a "middle button message". I expect it to work with your mouse. You can use MBUTTON to test this.
There is only a hope that your software can use this "middle button message". If not, ask the vendor how it should work.

Editing the registry:
ONLY EDIT IF YOU ABSOLUTELY SHURE WHAT YOU ARE DOING !
If you edit something wrong the whole system may crash.

start  REGEDT32.EXE
click on the window named:  HKEY_LOCAL_MACHINE
There you have e tree structure similar to th explorer. But you do not see directories and files, you see registry settings.
Navigate along the given path. If you have a serial mouse (connectet to COMx ) go to:

SYSTEM\\CURRENTCONTROLSET\\SERVICES\\Sermouse\\parameters\\

and double click on:     NumberOfButtons

change the value
exit REGEDIT32

I would like to hear if your specific software now regognized the 3rd button ...