Dear Morten, I am assuming that you selected New... from the menu or a context menu to create a new attribute?
I can't reproduce the problem...
Also, have you taken a look at MEGAN6 yet? It has been under development for some time and we will officially release it soon. I mention this because one of the features of MEGAN6 is a completely new sample viewer that should make adding attributes and working with metadata much easier... (available here: