You may have figured it out by now, but I'll try and help anyway. Don't worry, you're not the only one who's had trouble with the day sacks. It took me a while to figure out how it goes together.
You should have:
2x Small pouches
1x Yoke
2x webbing straps with male quick release buckles
You say you have already attached the pouches. If you've done it right, they should be joined to the yoke by 4 buckles along the top.
You now use the 2 straps with male buckles to join the female buckle at the end of each point of the V shaped yoke to the female buckle at the bottom of the pouch.
It should end up looking like a backpack and you wear it as such.
I have one in my loft from when I was in the Army Cadets.
If you don't understand my description, I can post some photos for you. Let me know if that would help.
The pouches don't attach directly to PLCE webbing, but the whole system is designed so that you can wear the two together without pouches or straps getting caught on each other.
Chris