What do you actually have in the table and what do you actually want to return?

As far as I'm aware there is no such function as TO_NUMBER in Access.

Isn't it actually an Oracle SQL function?

There are plenty of date/numeric functions that you could use.

Which to use is hard to tell without knowing what you have and what you want.

Here are a couple of examples, assuming your field name is mm.
Month as number to month name.
 
MonthName(mm) As MyMonth
 
Format("1 " & mm & "2011", "mmmm") As MyMonth
 
Month number from month name:
 
Month(DateValue ("1" & mm & "2011") As MyMonth
 
Month number from date:
 
Month(mm) As MyMonth
 
...