For whatever reason, this MySQL fails:

CREATE SCHEMA IF NOT EXISTS `partB` DEFAULT CHARACTER SET latin1 COLLATE latin1_swedish_ci ;
USE `partB`;

CREATE TABLE Employees ( ssn CHAR(11),
Name CHAR(30),
mlot INTEGER,
PRIMARY KEY(ssn))
ENGINE = InnoDB;

CREATE TABLE Dept_Mgr ( did INTEGER,
dname CHAR(20),
ssn CHAR(11) NOT NULL,
PRIMARY KEY (did),
FOREIGN KEY (ssn) REFERENCES Employees
ON DELETE NO ACTION)
ENGINE = InnoDB;

It provides the error:

ERROR 1005 (HY000): Can't create table partb.dept_mgr (errno: 150)

So what can be leading to this?

You need to specify the column(s) within the foreign table for that key:

FOREIGN KEY (ssn) REFERENCES Employees (ssn) ...

This command:

SHOW ENGINE INNODB STATUS;

is the friend if you have trouble creating foreign secrets. Output (abridged)

------------------------
LATEST FOREIGN KEY ERROR
------------------------
100225  2:51:42 Error in foreign key constraint of table test/dept_mgr:
FOREIGN KEY (ssn) REFERENCES Employees
ON DELETE NO ACTION)
ENGINE = InnoDB:
Syntax error close to:

ON DELETE NO ACTION)
ENGINE = InnoDB

Should you improve your statement to:

CREATE TABLE Dept_Mgr ( 
    did INTEGER,
    dname CHAR(20),
    ssn CHAR(11) NOT NULL,
    PRIMARY KEY (did),
    FOREIGN KEY (ssn) REFERENCES Employees(ssn)
) engine = innodb;

it will work.

You simply possess the foreign key referencing the table, not the column.

Try: references Worker.ssn