Incremental content plugin problems (not working)

Hi!
When I try to [add] something in the "Incremental Content Memberships" box on amember > admin > edit product page nothing happens. The only thing that works is [new] button which clears the fields when clicked.
I'm using Firefox 4.0.1 and I checked Tools > Error Console which displayed two errors:

1) load_links is not defined
Source File: x/amember/admin/products.php
Line: 1 (?!)
It's with every admin page, not just admin > products page.

2) $("#link_group_id")[0] is undefined
Source File: x/amember/admin/products.php?action=edit&product_id=1
Line: 538 (Code @line 538: 'link_group_id' : $("#link_group_id")[0].value,)

Additionally I have found more weird things. There is <form method="post" action="product_links.php"> in product.html file (in the plugin's folder). But what the heck is product_links.php ? There is no such file anywhere! I mean nowhere in the website's folders (incl. amember).
Another thing: There's "Link group:" field in the "Incremental Content Memberships" box before [add] and [new] buttons, but it has no form field after it. There's just an empty table cell. Oh, one thing does work. Adding groups. But it didn't change anything.

I even tried twice, reinstalled whole amember again, but nothing has changed, I'm still having exactly the same problems.

I was hired to create a membership site (with all the necessary scripts and software provided), so it's turned pretty frustrating at this moment. I would be grateful if anyone could help me.