As the FEATool OpenFOAM interface has been designed, tested, and validated for OpenFOAM version 5 on Linux systems, FEATool will first look for an OpenFOAM installation and directory in "/opt/openfoam5", next it looks to see if a "FOAM_APP" enviroment variable is set.
In your case neither seems to be the case, so you could either try to:
1) Make sure the FOAM_APP variable is set to the directory of your OpenFOAM installation
2) Make a symbolic link from "/opt/openfoam5" to the directory of your OpenFOAM installation
3) Install OpenFOAM version 5 in parallel any other versions