The track log I use is a GPX file (which can be exported from your GPS or even a GPS application on your iPhone). I use GPS Kit on my iPhone to track my movements and generate a GPX file which I can email or download. This will display a track once loaded into Lightroom that you can add images to.
So here's the odd part. I downloaded the GPX file to my desktop and loaded it into Lightroom, after which I deleted the file thinking that Lightroom would just copy the file into the catalog or directory. It does not copy it, if you delete it, Lightroom no longer knows about it and won't display it.
I set up a directory with my catalogs to put all my tracklogs. You can also put it into the directory that the images live in, load it and don't touch it. Either way, keep those tracklogs and don't delete them, they'll be gone for good.
P.S. I am LOVING Lightroom4. I'll be giving a lesson on it at the upcoming Bill Fortney His Light workshop down in Old Car City.