I found the answer :> This is part of the Nagios learning curve.
The plugin comes with the shell script wlsagent.sh which wraps up the wget command.
I copied the script to nagios/libexec directory. The following definitions look good in Nagios to monitor Weblogic server at port 9000.
# The 1st number after UsedMemory is warning threshold (%) for JVM heap usage; the 2nd number is critical threshold.
# These values are compared to the UsedMemory / HeapSizeMax ratio
define command{
command_name check_wls_memory
command_line $USER1$/wlsagent.sh 'hostname=$HOSTADDRESS$&port=$ARG1$&username=nagios&password=nagios&jvm=UsedMemory,80,90'
define service{
use generic-service
host_name
myhost.com service_description MEMORY USE
check_command check_wls_memory!9000
notification_interval 1440