You won't get any smnpwalk output from the MIBS installed on your system. When you do an snmpwalk on localhost, snmpget actually communicates with localhost to retrieve snmp data. The snmpdata is returned from localhost, regardless of the presence of the MIB file. Without MIB file you'd get the same values, but no names in the smnptree, just numbers like 188.8.131.52.184.108.40.206.220.127.116.11.1.1.
Actually, the MIB only translates the numeric strings to meaningful names to make it easy for us humans. Just try it, do a snmpget -v OID, and when you got the numeric address do the same with the numeric address.
MIB files are specified in /etc/snmp/snmp.conf (check snmp.conf and where they are on your system). This is an example:
Furthermore, if you have reason not to use this conffile, you can specify the MIB on the command line:
/usr/bin/snmpget -v 2c -m /usr/share/snmp/mibs/VISUS.MIB -c ****** 192.168.xxx.xxx VISUS::visus.hsv.\"RBE_RNW_NLANT:POWER.COMMON:STATUS.FUELLEVEL\"