Trying to hide #Error in a form field

There things to check
1. Correct mapping of your controls to the fields of your Recordsource
2. Your RecordSource so that is not producing #Error...

I suspect that you don't have the right number of parenthesis.  I usually test formulae like this in steps:
put in the DLookup, confirm that it's working;  surround it with the IsNull, confirm that it's working, then surround the formula with the IIf.  It's hard to test without having a connection to the data, but try this:

=IIf(isnull([txtEquipmentN]), "", (DLookUp("[Manufactor/EquipmentType]","tblEquipment","[EquipmentID] = " & [Forms]![frmSites]![txtEquipmentN])))

If txtEquipmentN is a text field, the Criteria portion of the DLookup needs single quotes around the reference:

DLookUp("[Manufactor/EquipmentType]","tblEquipment","[EquipmentID] = '" & [Forms]![frmSites]![txtEquipmentN] & "'")

It is a number field but this isn't working:

=IIf(isnull([txtEquipmentN]), "", (DLookUp("[Manufactor/EquipmentType]","tblEquipment","[EquipmentID] = " & [Forms]![frmSites]![txtEquipmentN])))

Am getting, The expression you entered contains invalid syntax.

Will the DLookup function work by itself?

Yes.  Perfectly.

I recommend saving all the values from controls to variables of the appropriate data type, so that you can display them for debugging purposes in the Immediate Window.  Then use the variables in your expression, rather than control references.

My solution worked perfectly.