Thanks Jerel4565 for your quick response.
Sorry for not being clear. Yes, I am using "nagiosadmin" as login name. And I just set up the "userup" for nagios and this is the output I get.
Error: Cannot open main configuration file '/home/Bille/userup' for reading!
Nagios 3.2.0 starting... (PID=5026)
Local time is Sat Jan 02 12:40:10 PST 2010
Bailing out due to one or more errors encountered in the configuration files. Run Nagios from the command line with the -v option to verify your config before restarting. (PID=5026)