You need to open a recordset against the table and use the AddNew method
Code:
Public Sub AssignValue()
Dim rst As DAO.Recordset
Set rst = CurrentDb.OpenRecordset("Attendancetbl", dbOpenDynaset)
If Me.txtpassword.Value = DLookup("password", "Employeetbl", "[username]= '" & Me.userCmbo.Value & "'") Then
With rst
.AddNew
!UserName = Me.userCmbo.Value
!TimeIn = Now() 'Assign value to Attendancetbl
.Update
End With
rst.Close
Set rst = Nothing
MsgBox "Time In Entered" & vbCrLf & vbCrLf & "Successfully Time In", vbInformation, "Congratulations"
txtpassword.SetFocus
End If
End Sub