Right Heres what i can provide information wise.
Description of the Problem: It seems like the cursor is having trouble selecting the trade post icon bottom when you click them in the new Trade window. Pop up information when you put your cursor over the icon flashes several times per second, but the cursor cant seem to keep the icon selected to keep the pop up information from staying up. It just selects it then deselects it as if your cursor was repeatedly moving off and on the trade post icon no matter where you place the icon(but repeatedly and several times per second).
Thinking about it is probably is a bug related to how the trade post icon pops up and overlaps with the holdings area. Don't really know the code or anything like that but consider the feature is new to the DLC and the fact you can easily select trade posts in the trade posts listings in your patrician family list in the Republic tab it makes me think that's what it is. Just my layman's opinion on it.
Workaround: I keep clicking frantically at the trade post icon until my clicks match up when the pop up information appears very difficult since like i said the pop up information flashes off and on several times per second. Eventually you can get the message to build a trade outpost.
This describes my problem well.
I am running this on a 2012 MacbookPro Retina
i7 2.7 Ghz
16 GB Ram
1GB Vram, Nvidia GT650M
OSX 10.8.2
Checksum XJSG