Adding Constraints

I cannot figure out how to add a constraint to an existing table.

For example, I have a table with an "id" field that should have been created as the primary key but was not. The documentation seems to indicate that I can do:

ALTER TABLE MyTable MODIFY(id INTEGER PRIMARY KEY);

but this results in:
  ERROR at line 1:
  ORA-02253: constraint specification not allowed here

In other cases I want to add a REFERENCES constraint, but they all produce an error similar to this.

Thanks for your help!

--
Bob Swerdlow
Chief Operating Officer
Transpose, LLC
rswerdlow_at_transpose.com