Problem Synchronising Unbound Combo Boxes

I'm pretty hopless with VB but I am trying very hard (hours spent on this forum reading code I only slightly get the gist of) to implement a form that takes input from two synchronised unbound combo boxes and then displays records matching the input in a sub-form in datasheet mode. I'm failing at the first part just now: the combo boxes. No doubt I'll fail at the second part too but I'll deal with that when I get this fixed. :)

I was using the northwind DB to learn how to synchronise the combo boxes. I successfully did it by following the instructions and using this code from MS kb article 209595 in the After Update procedure:

Me.Products.RowSource = "SELECT ProductName FROM" & _
" Products WHERE CategoryID = " & Me.Categories & _
" ORDER BY ProductName"
Me.Products = Me.Products.ItemData(0)

I then tried to implement this on my database by changing the relevant information to reflect my database info. as shown below, and it does not work. I switched Product references for my Unit references and Category references for my Item references but I don't think I got it right or over-simplified it due to the table structure.

Private Sub Item_AfterUpdate()
Me.Units.RowSource = "SELECT Unit FROM" & _
" Units WHERE ItemID = " & Me.Items & _
" ORDER BY Unit"
Me.Units = Me.Units.ItemData(0)
End Sub

When I compile the code it stops at Me.Items with the error "Method or data member not found". So I assume that I have not referenced the right objects when I modified the code. I thought this was maybe due to the two tables I'm using being in a many-to-many relationship via a junction table, as opposed to the northwind tables which are directly linked by a one-to-many relationship. If this is indeed the case, can anyone help me put this right. I have attached my table structure / relationships in a .jpg to this post.

Any help from all you smart developer dudes would be muchly appreciated.