If you enable the textobject plugin text files (see the plugin doc what suffixes are supported) are used like images or real videos. So you put it into an album like you do with images or real multimedia files.
The textobject content is simply displayedand if this is embed code of a service that is displayed. Theoetically you can put several youtube videos into one file or even mix. Whatever you like. It was originally not made for this use. but it is a nice trick to manage external videos Zenphoto otherwise cannot naturally being file system based.
To have proper thumbnails for these follow the instruction for video thumbs.
A demo of this is the screencasts album within the screenshots on our site.