How to I open up the Cheat console in this game?

If you've already edited the "autoexec.cfg" file in the game's "Main" directory to include the line set developer 1 you just have to press ` during play (it's the key directly below the Esc key).

Hope that helps!

How do I get there on Windows 7?

computer/C drive/program[86]/gog/kingpin/main/autoexec.cfg

That's how to get there, but how to type anything in to the file? Can't add the line 'set developer 1'.
What's the trick?

Finally figured it out: right click the file, click properties, add 'set developer1'.

But when l click on the Kingpin icon and start the game, the "~" key still does not activate the console.
Am completely at a loss.

Does anyone have the answer?

Same here Sam :(

I didn't mess around withe the autoexec.cfg file. I've played this since the game was new and never had a problem. The desktop shortcut that the Gog installer creates doesn't seem to work with it so I created my own shortcut. You need to add + developer 1 at the end of the target line. It should look like this;
"C:\Program Files (x86)\GOG.com\Kingpin\kingpin.exe" +developer 1

If you did it like that, you need to put a space between the 1 and developer.

If not, I had a problem with a certain model of Gateway that installed a little thing in Device Manager called "eHome Infared Transceiver" that disabled the "`" key for any program. I remember that I had to uninstall the driver EVERY SINGLE TIME that I booted up the computer.