Looking to try and make some characters with the new subclasses from HotFL but found out that it wasn't put out for the downloadable WotC builder.
What's the easiest way to add the new elements?
This is only possible with a tool of questionable legality. Some fans have made a Character Builder loader called CBLoader that allows you to load custom XML files into the Character Builder.
Given this site's discomfort regarding tools that are heavily used for piracy, I won't link to the Character Builder loader. If you own the Heroes of the Fallen Lands material, converting it probably doesn't violate copyright law, but it might violate your license to use the software or the material.
We can't help you
The Downloadable character builder was essentially the beta. It was discontinued, and unsupported and since the live version launched on the D&D website. The Character Builder hosted on D&D is up to date and kept that way through D&DInsider subscriptions. Us telling you how to replicate content in it would be copyright infringement. We avoid advocating piracy on our site so that we can quote snippets of rules from books verbatim without having companies trying to shut us down.
See our policies on this stuff:
I suggest you subscribe to D&D Insider and use the online character builder.