Are you Oneworld Sapphire or higher and therefore trying to get seat assignments for free?
If so, you may have better luck using the Royal Jordanian or Malaysia Airlines websites. Go to their Manage Booking section and enter your IB booking reference and you should be able to select seats. Then go back to IB website and see if they show assigned seats. Check again the next day to make sure they haven't disappeared. If they have, go back to RJ or MH, delete your FF number, then re-enter it and select seats again.

If you are not Sapphire or higher and need to pay for seat assignments, you have to do this with IB. Their IT is terrible so you may need to call. US number is 800-772-4642. They have some really useless agents, and some really good ones; hit or miss which you get!

Good luck.