on the mexico side.  You will find at least two areas next to the pedestrian line where you will see people getting bikes.  It's $5 on weekdays and $7 on weekends.  Once you have the bike you ride/walk the bike next to the curb following along the pedestrian line but passing everyone.  Near the border crossing entrance you will find the line for the bikes, it's usually fairly short.  Once they tell you to go, there are signs that lead you through the rest of the line.  Once you are past the customs desk and out of the building on US side there is a man instructing the bikes were to go.  You walk up about a block and drop the bike off.  Piece of cake, I even saw a women with a long dress grab a bike, it's easy enough to walk them, in fact some are in such bad shape you pretty much have to.