This should work for the IIf() expression:

   =IIf([Type of employment]
               In ("Company", "Self employed (External)"),
           "Not Applicable","Please Press Browse Course")

I had to break it onto multiple lines for the newsreader, but it would
really be one line.

But you should be aware that binding your text box to an expression like
this will make it uneditable.  Was it your intention that the user
should enter something in this text box if [Type of employment] is "LSBU
employed (Internal)", and that otherwise the text box should be
disabled?  If so, you shouldn't bind the text box to an expression.  You
might use code in the form's Current event and in the AfterUpdate event
of [Type of employment] to set the text box's Enabled property.
Something like:

'----- start of example code -----
Private Sub EnableDisableCourseCodes()

   Me!CourseCodes.Enabled = _
       (Me![Type of employment] = "LSBU employed (Internal)")

End Sub

Private Sub Form_Current()

   EnableDisableCourseCodes

End Sub

Private Sub Type_of_employment_AfterUpdate()

   EnableDisableCourseCodes

End Sub
'----- end of example code -----

I don't know if that's exactly what you're after or not, but it rather
sounded like you want something like that.