Why is MySQL Workbench telling me I need a semicolon?

The problem is the delimiter, you should change the delimiter. I hadthe same problem and changing the delimiter solved the issue. Refer to @Alex Answer.

Here is an simple explanation, quoting MySQL documentation:

The example uses the mysql client delimiter command to change the statement delimiter from ; to // while the procedure is being defined. This enables the ; delimiter used in the procedure body to be passed through to the server rather than being interpreted by mysql itself. See Section 22.1, “Defining Stored Programs”.

https://dev.mysql.com/doc/refman/5.7/en/create-procedure.html

DELIMITER // 
create procedure some_procedure() 
# Do what you need here

END // 

DELIMITER ; # change the delimiter back again.

I think the problem is : you are not using DELIMITER.

So just put it this way:

DELIMITER //
create procedure AddColumnUnlessExists(
    IN dbName tinytext,
    IN tableName tinytext,
    IN fieldName tinytext,
    IN fieldDef text)
begin
    IF NOT EXISTS (
        SELECT * FROM information_schema.COLUMNS
        WHERE column_name=fieldName
        and table_name=tableName
        and table_schema=dbName
        )
    THEN
        set @ddl = CONCAT('ALTER TABLE ', dbName, '.', tableName, ' ADD COLUMN ', fieldName, ' ', fieldDef);
        prepare stmt from @ddl;
        execute stmt;
    END IF;
end //
DELIMITER ;

EDIT https://dev.mysql.com/doc/refman/5.7/en/stored-programs-defining.html

If you use the mysql client program to define a stored program containing semicolon characters, a problem arises. By default, mysql itself recognizes the semicolon as a statement delimiter, so you must redefine the delimiter temporarily to cause mysql to pass the entire stored program definition to the server.

To redefine the mysql delimiter, use the delimiter command. The following example shows how to do this for the dorepeat() procedure just shown. The delimiter is changed to // to enable the entire definition to be passed to the server as a single statement, and then restored to ; before invoking the procedure. This enables the ; delimiter used in the procedure body to be passed through to the server rather than being interpreted by mysql itself.