If Nz(DLookup("[AccessLevelFieldNameHere]", "UsersTableNameHere", "[UserIDFieldNameHere]=" & Chr(34) & VBA.Environ("username") & Chr(34)), vbNullString) = "Admin" Then
DoCmd.OpenForm "FormNameHere"
Else
MsgBox "You are not authorized to open this form.", vbInformation, "Error"
End If