I have the same situation in another page I'm doing.
The thing is that I can create a "lookup" page for a field, and it works perfectly.
The problem I have now is that there is one more field in the page that needs to be filtered depending on the first one.
So lets say for example, I have a field called "Item Group". There are National Item Groups and International Item Groups. I want the field in the page to display only National Item Groups, so I create a "lookup" page called "National Item Groups", and set it as lookup page for the first field.
Now the users selects one of the National item Groups, for example "seeds". I then have another field in my page wich is "ItemID".
¿How can I make this field to show only Items that are "seeds"?