Apparently, from the description, the build.properties file is supposed to be part of the library template.
It is a file used to describe the library, allowing to import it automatically from Processing and to build a listing of contributed libraries.
And they describe how to move the build.xml, so I don't know what isn't working for you.

Note: I haven't used this template, yet... Made a library by hand...

It should be in the resources folder, along with the build.xml file. There could be a chance that you're using an older version of the library template that doesn't include the build.properties file, and has you directly modify the build.xml file. If this is the case, then download the latest library template from the website.

Thx calsign, i used the wrong  library template. But now after building i cannot find the ' distribution' folder.