I just made a simple game and added 4 objects - backpack, apple, banana, and HK's head. All you need to do, I think, is make the backpack, click on the features tab, and click on 'container'. I then clicked on the boxes for can be open, can be closed, and list children when object is looked at or opened under the container tab. The commands 'put apple in backpack', etc. worked fine. When I looked at it when open, it told me the objects that were in it.
I added the code view of it below.
<object name="backpack">
<inherit name="editor_object" />
<inherit name="container_open" />
<look>It is a backpack.</look>
<alt type="stringlist">
<value>pack</value>
<value>bag</value>
</alt>
<take />
<feature_container />
<listchildren />
</object>
<object name="apple">
<inherit name="editor_object" />
<look>It's a red apple.</look>
<take />
</object>
<object name="banana">
<inherit name="editor_object" />
<look>It's a yellow banana.</look>
<take />
</object>
<object name="HKs head">
<inherit name="editor_object" />
<alias>HK's head</alias>
<look>It is the ugliest thing you have ever seen. lol</look>
<take />
</object>
</object>
<object name="B">
Hope that helped.