As long as you didn't name the module ConvertTime, that should be all you
need to do.

Actually, though, I just noticed there are some errors in my code. (I made
some changes to variable names, and didn't propagate the changes
everywhere). The function should be:

Function ConvertTime(InputValue As String) As Single
Dim intDaysP As Integer
Dim intHoursP As Integer
Dim intDays As Integer
Dim intHours As Integer
Dim intMinutes As Integer

  intDaysP = InStr(InputValue, " Days ") + 6
  intHoursP = InStr(InputValue, " Hours ") + 7

  intDays = Val(InputValue)
  intHours = Val(Mid(InputValue, intDaysP, 2))
  intMinutes = Val(Mid(InputValue, intHoursP, 2))

   ConvertTime = CLng(intDays) * 24 + _
       CLng(intHours) + _
       CLng(intMinutes) / 60#

End Function

Actually, if you're going to be using this with a table, where it's possible
that fields you're passing to the function might contain Null values, you
should probably change that to:

Function ConvertTime(InputValue As Variant) As Single
Dim intDaysP As Integer
Dim intHoursP As Integer
Dim intDays As Integer
Dim intHours As Integer
Dim intMinutes As Integer

  If Not IsNull(InputValue) Then
     intDaysP = InStr(InputValue, " Days ") + 6
     intHoursP = InStr(InputValue, " Hours ") + 7

     intDays = Val(InputValue)
     intHours = Val(Mid(InputValue, intDaysP, 2))
     intMinutes = Val(Mid(InputValue, intHoursP, 2))

      ConvertTime = CLng(intDays) * 24 + _
          CLng(intHours) + _
          CLng(intMinutes) / 60#
  End If

End Function

To use this in a query, simply put ConvertTIme([MyField]) in the query.
(replace MyField with the actual field name)

As to your other problem, when you say "ElapsedTime is my field formated
with 00 Days etc...", are you saying that you've created a module that has a
variable named ElapsedTime in it? How did you declare the variable? The
function is expecting it to be declared as a string: if you're not
explicitly declaring variables, ElapsedTime would be a variant, and you'll
get that problem. The second version of the function above will solve that
problem.

If you're not explicitly declaring variables, you should be. If you haven't
got Access set up to force you to declare variables, so that it
automatically puts "Option Explicit" at the top of each module, you should.
(you do this on the Module tab through Tools | Options while in the VB
Editor). Doing so will vastly decrease the amount of time you need to spend
on debugging!