I just tested it and it works fine, key is to highlight first and then only single-tap the word.
Procedure:
1. Double-tap the word to highlight. A small menu comes up with dictionary, highlight, etc.
2. Hit the highlight icon - most left one in the small menu on the bottom
3. Single-tap the (now) highlighted word. A popup comes up asking 'By drawing' / 'By keyboard'.
4. Hit 'by keyboard'
5. Use the keyboard to type away, hit 'Done' when ready.
6. Single-tap the word again to view / edit / delete the annotation (
example).
This is of course the European version (with the latest firmware) but I very much doubt it works any different on other 600's.