Enable SNMP on Mac OS X
First of all, delete the duplicate rouser line from snmpd.conf, leaving just the "rouser rouser priv" line.

Second, note that you created your user with AES:

sudo net-snmp-config --create-snmpv3-user -ro -a 12345678 -x 12345678 -X AES -A SHA rouser

But are trying to walk using DES:

snmpwalk -v 3 -u rouser -l authPriv -a MD5 -A 12345678 -x DES -X 12345678 localhost .1

That's just not going to work. Swtich to -x AES in the snmpwalk, and there's a much better chance it will work. ;-)