Lets start here
Code:
Error: Service check command 'check_xi_service_snmp' specified in service ' ... not defined anywhere!
This says you don't have the relevant check cmd installed (or the path doesn't see it).
In my std nagios install, check_... progs are in /usr/local/nagios/libexec, you should have a similar dir somewhere.
Code:
has no default contacts or contactgroups defined!
On my system /usr/local/nagios/etc/objects/contacts.cfg
On a more general level, you need to check nagios.cfg (/usr/local/nagios/etc on my system)
HTH