It's not listed in my Application Manager. I tried refreshing and everything else but can't seem to get any additional apps to show up.
Quite possibly the problem here is that you don't have the right repository in your Application Manager. I think OMWeather is in the extras-devel repo.
In Application Manager, go to Tools->Application Catalog...
Create a new entry
Set the Web Address to http://repository.maemo.org/extras-devel/
Set Components to "free non-free" (without the quotes)