I gues not, since the config file isn't executed as a normal aspx page. It's the same as classes: you can't use request/response and here I guess the server variables are also unreachable.
Try putting it into a class file like this:
Public Class Storage
Public ConnStr As String
Public Function Initialize()
ConnStr = "Provider=Microsoft.Jet.OleDb.4.0;Data Source=" & Server.MapPath("admentornet.mdb")
End Function
End Class
Now when you need the ConnStr value, just use this:
Dim ReqStorage As New Storage
ReqStorage.Initialize()
Dim ConnStr As String = ReqStorage.ConnStr
Of course a config file is better, but in an emergency you can use this. You'd probably already know it, but still, I use it, since the data is compiled, wich is more secure than a config file I guess.