In any case, your foobar2000 directory should have a subdirectory called components.
Just put the DLL in there. And the license file if you want. Obviously that doesn't matter for the DLL's functioning, so you can also rename it to something specific if you want, especially if you plan on putting other add-ons in there later.
Once you've done that, I'm not exactly too sure. Let me try downloading this plugin.
Once you've put it there, opening up foobar2000 should cause it to pop up a dialog box saying that there's a "New user interface module(s) found", and asking you which one you want to use. Just pick the Columns UI and click OK.
Comments
In any case, your foobar2000 directory should have a subdirectory called components.
Just put the DLL in there. And the license file if you want. Obviously that doesn't matter for the DLL's functioning, so you can also rename it to something specific if you want, especially if you plan on putting other add-ons in there later.
Once you've done that, I'm not exactly too sure. Let me try downloading this plugin.