To install FHEM into /opt/fhem you have to take the following steps:

1. Switch to root mode:
   $> sudo su –
2. Get archive key and add it to system keys:
   $> wget -qO – http://debian.fhem.de/archive.key | apt-key add –
3. Add reposetory to your system sources:
   $> nano /etc/apt/sources.list and add the following line to the bottom :deb http://debian.fhem.de/stable ./
4. Update sources:
   $> apt-get update
5. Update system:
   $> apt-get upgrade
6. Install FHEM:
   $> apt-get install fhem
7. This will install all dependencies an FHEM on its own. (I you’re ask you have to type Y -> it’s not really a choice)

Now you can open http://192.168.108.15:8085/fhem to get to your FHEM installation (replace 192.168.108.15 with your PIs IP ;-))