codecs for Win 7 DVD Maker?

I've installed the most recent pack, and it works fine for Windows Media Player, but how can I get it to work with Windows DVD Maker?
I'm trying to convert .webm and .mp4 files to standard DVD.

.mp4 should work fine as that is a format supported by Windows. These Microsoft tools never work reliably with unsupported formats.

I tried a .webm file and that indeed gave an error. I don't think there is a solution for that, other than converting the file to a supported format.

If your objective is just to convert the files to a DVD, without any video editing, then you could try an alternative tool such as DVDStyler.

For now, I would settle for the "every X minutes" chapter settings imposed by DVD Maker. Ultimately, I'd like to find a good authoring/burning program (hopefully freeware?) where I can put in custom chapter markers (at specific frame or time code marks).

I know conversion does detract somewhat from the quality, by its' very nature. The webm plays back quite nicely in WMP, so I'm trying to find a way for DVD Maker to look at K-Lite, read the file and go. I could live with converting if the quality is high enough, but I'll need something that detects if codecs are already here, rather than installing yet another pack.

Microsoft is annoying... if WMP reads it, then everyone should (DVD Maker, Live Movie Maker, etc).

DVDStyler should support creating chapters.
http://www.dvdstyler.org/en/