Incorrect parameter count in the call to native function 'ISNULL'

I would suggest that you switch to the ANSI standard function coalesce():

(dbo.OHEM.lastName + ', ' + coalesce(dbo.OHEM.middleName, '') + '' + coalesce(dbo.OHEM.firstName, '')
) AS EmployeeName

You could also make your query easier to read by including table aliases.

EDIT:

As a note, I seemed to have missed the direction of conversion. The MySQL query would use concat():

CONCAT(OHEM.lastName, ', ', coalesce(OHEM.middleName, ''),
       coalesce(concat(' ', OHEM.firstName), '')
      ) AS EmployeeName

The MySQL equivalent of ISNULL is IFNULL

If expr1 is not NULL, IFNULL() returns expr1; otherwise it returns expr2.

Maybe also look at SQL NULL Functions

The ISNULL from MySQL is used to check if a value is null

If expr is NULL, ISNULL() returns 1, otherwise it returns 0.