Little correction, the bridge's .p3d name is most_stred30.p3d, located just as Yamato said under O\Hous\.
And you can change the height in objects by selecting it, and clicking "Poloha Objectu" (Move) Button in tool bar, it opens new menu where you can set the Elevation relative to surface, not sure if you meaned that with Absolute height though...
I think there may be another way of placing bridges, so far i have tested to add bridge part to the special parts section in "Defination of Road MyRoad" menu, then you can add bridges to your road just like terminators & crossroads, only problem is that it connects to the road perfectly, but its like 10 meters above the main road... And you cant change its relative via the "Poloha Objectu" (Move) Button, as it is a road... But you can change the height by selecting it and right click on it, but there we come up to an another problem, because its a road, relative cant be under 0, so it doesnt accept any numbers starting with - (minus)... So you can only move it higher...
So, does anyone have any tips & tricks for this bridge thing? Or is the only way just made it "manually" using artificial objects as Yamato said?