Click the Design tab, then the arrow next to the theme gallery so the
gallery opens. Select "browse for themes." Change the file type to "all
files," and navigate to your POT file. Or leave the file type as it is, and
navigate to a THMX or POTX (or PPTX or PPSX, etc.) file.
Or just click one of the theme thumbnails in the gallery on the Design tab.

Signature
Echo [MS PPT MVP] http://www.echosvoice.com
What's new in PPT 2007? http://www.echosvoice.com/2007.htm
Fixing PowerPoint Annoyances http://www.oreilly.com/catalog/powerpointannoy/
> How can I apply a template to an existing presentation in Powerpoint 2007?
> It used to be a matter of clicking design and selecting one from the
> sidebar.
> Now I seem to have to start with a whole new presentation. What on earth
> have they done?