You can reach Merisani-Valcele Dam on the way from Pitesti towards Curtea de Arges, downstream from Zigoneni Dam and upstream to Budeasa Dam.
Near one of the parking places on the lake shore, if you climb the surrounding hills, you will find a geocache.
I used the GPS of two smartphones so coordinates might not be 100% accurate. Please post new coordinates if necessary.
Good luck!