Your CurrentForm is a different type than the Form you put in the Hash Table. One is type Form1, the other is type frmAdditemNumber. The following code works fine:

Dim MyForm As New Form1
Dim ht As New Hashtable
Dim CurrentForm As Form1

ht.Add(1, MyForm)

CurrentForm = ht(1)

CurrentForm.Show()

And you can even leave out the CurrentForm and do:

Dim MyForm As New Form1
Dim ht As New Hashtable


ht.Add(1, MyForm)

ht(1).Show()