Problem: Unity "steals" the Alt-key so that it doesn't function as it should in Inkscape.
- install dconf-tools
- launch it
- go to org -> gnome -> desktop -> wm -> preferences
- set "mouse-button-modifier" to "<Super>" (without the quotation marks).
DO NOT SET IT TO "none" or leave the field empty, or you won't be able to left-click normally anymore anywhere in Unity (I know because I made the mistake, had to log out and log in to Xfce to correct it, Unity was totally messed up).
(With thanks to this post but take caution not to set the value to "none"!)