Two ways:

Hard: Create/register a COM interface/category/class factory that will contain the interface used to activate the plug-in. Then use COM API calls to enumerate the plugins available on the system and CoCreateInstance to instantiate them.

Easy: Specify a particular directory for plugins and specific exported functions to activate the plugins. Then use LoadLibrary(Ex) to load the libraries.

I'm working on an app that I would like to make extendable with plug-ins in dlls. My question is how do I get my app to look for and load dlls that it doesn't even know are going to be there?

you can get all the files in a directory by using FindFirstFile and FindNextFile, then try to LoadLibrary on all. i recommend just exporting functions, writing a COM interface is too much, unless you want to make it scriptable with JavaScript, VBScript with IActiveScript