as above with RD's comment, then click the icon to choose one that you want to use, put evolution or whatever you want to call it, the only possibly tricky bit is the command. Some will just work with the name (Opera does that), but some you will need to put a "path" to where the executable file is. Something like /usr/bin/evolution (which might work, I don't know, as I don't use evolution).
Or if it (evolution) appears in the menu, you can always put a "panel" on the task bar - you might have to move stuff to the right to make room, but right click and then add to panel>application>internet>email>evolution should do the trick and you get an "evo" start button on the task bar (with KDE of course, don't know with gnome, don't use it, never have got on with it!)
"What the large print giveth, the small print taketh away". Tom Waits