DME arcs in an airliner can be flown two ways. First, the DME arc can be coded in the FMC data base if it is part of a published instrument approach procedure. Then the crew just uses LNAV and follows the ARC which was loaded from the data base. Second you can creat your own unpublished DME arc by using the format ABC010/10 and insert it into the legs page to create your own arc. ABC being the reference VOR, and 010/10 being the radial and distance. For example if I wanted to start a 10 DME arc from the LAX VOR on the 040 radial and exit on the 090 radial I would enter LAX040/10, LAX050/10, LAX060/10...LAX090/10 in the FMC, then use LNAV to fly the arc.