Hello,
First, make sure you have checked the box for 'Permit use via context variable reference' when creating the stored password. To look this up dynamically:
1) Create the stored password
2) Add a 'Look Up Context Variable assertion where the variable ${user} is set to the stored password name
3) Reference the output of this assertion, ie: ${lookup.output}
This will display the password for the user specified.
Regards,
Joe