I was worried about finding the barrage because the directions were quite vague, but thats because it's easy!
Take the last right before the rosso ferry. when you miss it (because you will!) drive round the hawkers and turn round at the garage opposite the ferry terminal and go back less than 100 metres, now it is the first on your left! the track follows the river and you criss cross around to find the best route. you will come to an industrial building after a while, go to the right of it, although if you get it wrong guards will call you back and point you in the right direction. eventually (after about 60km) you reach a T junction. turn left. this rd is dreadful. really bad corrugations, but don't be tempted to whizz along here as there are also bad washouts. my mate bent a wheel here, which takes some doing on a landcruiser (make sure you use steel wheels so you can hammer them back into shape again, and a 2kg lump hammer) all in all it is about 80km and will take 3-4 hours (although we did it at night which slowed us down a bit)
yu will go through border formalities and pay a fee to cross the barage. it seems expensive for what it is, but is cheaper than the ferry. once you have crossed the barrier, if memory serves me correctly (and don't quote me on this!!) you go to the police in a hut on the right. then you go to the customs guy in a hut on the left, it is here that you negotiate for the lassez passer.
one final thing, watch out for the police in St Louis, they are the original bastards!! we were stopped at least 5 times to check we had fire extinguishers and warning triangles, also because we had taped over our windows, for having a non operationg headlight, an alleged paperwork infringement and alledgedly going the wrong way up a one way street (like all the locals did, and there were no signs!!). It is a shame because I have visited st louis before and it is lovely and I wanted to show it to my friends, but in the end we couldn't get out quick enough. we only paid one bribe, to a copper who clearly enjoyed his violence!! with the others we just toughed it out, spoke minimal french and played dumb.
1990 Landcruiser H60. Full rebuild completed 2014
2012 Hilux, mountain top and bedrug with GG AT2s