1 You need to have access to the server where Wm IS is running to be able to perform the suggested solution.
2 Webmethod IS stores all user authentication/authorization details in a plain XML file -- [WM_HOME]/config/users.cnf .
3 All password is hashed based on the value of user name and its password, simply copying and pasting a hashed password from other user name will not work.

1 Obtain the default password for Administrator
     <>Shutdown IS
     <>Rename users.cnf to users_original.cnf
     <>Restart IS
     <>The IS will recreate a new users.cnf which stores loggin details for all default users
     <>Open the file and search for "Administrator", you will see xml tags similiar to the following
         <record name="Administrator" id="15" javaclass="com.wm.app.b2b.server.User">
                     ***Other stuff here***
             <value name="passHash" id="17">sha1vI5TC3V31VdoYdyukqpb7AYXHXQ=</value>
                     ***Other stuff here***
         </record>
     <>Task a copy of the hashed value embedded in the xml tags above     

2 Restore the default password for Administrator
    <>Shutdown IS;
    <>Open users_original.cnf and find the entry of Administrator;
    <>Replace hashed password with one you just copied from default user.conf;
    <>Make sure you has not touch any other XML entries in the file;
    <>Save and close the file;
    <>Rename users.cnf to users_default.cnf (could just delete the file if it is in no use);
    <>Renmae users_original.cnf back to users.cnf;

3 Test
    <>Restart IS;
    <>Try to access the IS Admin console using Administrator and the default password;
    <>It should work now;