I think I found a bug in Bonita BPM community edition 7.6.0:
1. Create a send message event
2. General -> Messages -> Add
3. Message content -> Add
4. In field "Value", there are no options shown.
5. When you click on the pencil symbol, there are no expression types shown in left column. Normally here shoud be the following options: Constant, Java, Parameters, Script, Variable.
In Bonita 7.5.4 this is working.
(Sorry for my bad english)