A LINUX How-to for the XS++ 4.1 Beta. (credits go to GarfielDD from SE-NSE)
Run these commands in the terminal.
Code:
aptitude install libfltk1.1
aptitude install gksu
chmod +x /your/path/to_xs/XS++ v4.1
gksudo /your/path/to_xs/XS++ v4.1
EDIT:
Alternately, you can search for libfltk1.1 in the package manager of your distro and install it and then run XS++ from the terminal.