I'm on Win7 and had the same problem. PDF24 didn't show up as a printer! Here's how I solved it:
First, I didn't have the Print Spooler service running. Open Control Panel>Administrative Tools>Services. Scroll down to Print Spooler, and if it's listed as Started and Automatic you're fine. Right click to set startup to automatic and/or start the service if you need to.
Now I still didn't have PDF24 as a printer and I still got the same error. So I uninstalled and reinstalled somewhere other than Program Files (which sometimes doesn't give permission to programs to do things that you want them to do). I chose C:/Work/PDF24 etc.
With the new installation I checked my printers available in Control Panel>Devices and Printers. I went ahead and set PDF24 to default by right-clicking and choosing that option.
You might be able to get it to work without all of these steps. For example, it might not be necessary to install somewhere other than Program Files. But if you follow my steps it'll catch all of the problems that I had at least.