I added a new article to my Knowledge Base today and created a tag for it. I then decided I wanted to add child tags to the newly-created tag. I had no trouble doing so in the tag tree, but I could not figure out a way to add the child tag to the article. I could remove the parent tag from the article, but when I tried again to add tags, Clibu showed me only the parent tags, not the child tags. Perhaps I'm missing something obvious, but I thought I'd post this in case it really is a bug. (FWIW, I'm using the Clibu Beta on Win 7 64-bit)
When you click on 'Add Tags' in an Article you should see a list of all Tags in a flattened structure, so Parent: Car -> Child: Ford will be displayed as 'Car/Ford'. You can select this by clicking on it or click the checkbox beside it and then click 'Apply' to add it to the article. You can also type 'Ford' to see only tags that include that word at some level in the tags hierarchy. I hope that resolves this issue.