You can't currently do exactly what you want, though the feature request has come up often enough that you can expect it to find its way into a future build.
Currently, the easiest way to accomplish something similar to what you want will be to go into the html editor of the text editor (denoted by the '<>' symbol at the bottom left of the myUdutu text editor), and modify the html directly to get a similar result. For example, if your text was:
The quick brown fox jumped over the lazy dogs.
You could go into the html editor, and modify it to read:
The quick brown <span title="alert carnivorous mammal with pointed muzzle and ears and a bushy tail"><font color=#0000ff>fox</font></span> jumped over the lazy dogs.
The word 'fox' would be blue, and when you moused over it, the definition would pop up in a tooltip.
I understand this is a little more of an advanced topic than you probably had in mind, though we do intend to expand upon glossary functionality down the road to make it simpler.