This is how I filter records to show a specific plant, selected from a combo box:
' Dim these fields
Dim theFilter As Variant
Dim thePlant As Variant
'then in your filter button click event
'you would want to add error routines, etc
thePlant = Me.cboPlant
theFilter = "[Plant] = '" & thePlant & "'"
DoCmd.ApplyFilter , theFilter
'I'd also recommonend building a button to show all the records again. The code on its click is
DoCmd.ShowAllRecords