[LITMUS^RT] Creating a new plugin problem

[Matteo]

Hi everyone, I want to develop a new plugin for LITMUS^RT, so I 
downloaded the virtual image found on the website and I started to 
follow the tutorial that I found on the wiki. I'm currently stuck on 
step 2: I created the sched_demo.c file (copying the code from the 
tutorial), added sched_demo.o to the obj-y list in the Makefile located 
in the litmus folder and compiled the kernel using make in the litmus-rt 
folder. When I try to run the instruction setsched DEMO, it returns me 
this error:

Error: Cannot write to /proc/litmus/active_plugin. Do you have root 
privileges?
Error: Setting new plugin failed.
Are there any active real-time tasks?

Can you tell me what I am doing wrong? Thanks.
Matteo Locatelli