It sounds as if you're wanting a Format instead of an Input Mask. On the
Format tab of the Properties dialog, set the format to Currency.

If you want to type in the value without typing the decimal, in the
textbox's AfterUpdate event you could divide the entered value by 100 to add
the decimal.

Example:
Me.txtMyTextbox = Me.txtMyTextbox / 100

If you wanted to check for the user typing a decimal, you could check for
the existence of the decimal in the text before the update and set a form
level flag to prevent the divide by 100 if the decimal was entered by the
user.

Example:
Option Compare Database
Option Explicit
Dim bolFlag As Boolean

Private Sub txtMyTextbox_AfterUpdate()
If Not bolFlag Then
   Me.txtMyTextbox = Me.txtMyTextbox / 100
End If
bolFlag = False
End Sub

Private Sub txtMyTextbox_BeforeUpdate(Cancel As Integer)
If InStr(Me.txtMyTextbox.Text, ".") > 0 Then
   bolFlag = True
End If
End Sub