Sometimes you need to pack the ipks in the old tar format, so that ipkg kann find them.
Unpack the package as described here:
http://www.openzaurus.org/oz_website/faq/faq?id=241And pack it again as described above. The problem comes now if you have a newer tar, as I have on my debian sarge or on Debian Sid. Then you have too pack it:
tar --format=oldgnu -czvf ....
Before packing again, you could have a look also at the contol file in the package. Sometimes there are stories about blank lines confusing ipkg.
You could also try an other ipkg package, that can handle the portabase. But I dont know, if there is one and which version.
Cheers,
Sam