You need Where as a string:
[tt]stDocName = "rptEWOReport"
DoCmd.OpenReport stDocName, acPreview, , "ProcessStep = " &[Forms]![frmEWOMenu]![cmbEWOProcessStep][/tt]
If ProcessStep is a text field, you will need single quotes:
[tt]stDocName = "rptEWOReport"
DoCmd.OpenReport stDocName, acPreview, , "ProcessStep = '" &[Forms]![frmEWOMenu]![cmbEWOProcessStep] & "'"[/tt]