I created a new C# project in Visual Studio (just started VS again, and created a new project). In there, I created the customevents, and built it.
Then back to the initial site (TheBeerHouse), I clicked on Website, Add Reference, and refered there to the project I just created. Finally, under Project Dependencies, I set the BeerHouse to rely on the CustomEvents one.
No worries, it took me quite some time to figure it out first, too. And then yesterday I finished up chapter 4, and only then realised that I started working on an older version of my code, which didn't have the project reference yet :(