Ditto what
@Bluejets said in his post #2. More information is BETTER than less information when you are asking strangers for help. Use a program like Microsoft Paint to reduce the size of high-resolution photographic images until they will upload successfully to this site. Placing a ruler next to the "little rubber pieces," temporarily secured with masking tape, may help us identify where the arm mounting holes are.
Check out the professional monitors on
this ASUS website page and then tell us EXACTLY which ones you want to mount.
Have you tried to contact the vendor who supplied your monitors?
Did the monitors come with any sort of printed instructions?
The "little rubber pieces" are probably "fillers," either to keep dust and detritus out of the monitors until they are securely mounted, or to protect the threaded nut inserts molded into the plastic case of each monitor. You can TRY to gently pry them out by grasping them with a pair of long-nosed pliers, but I am only guessing at this procedure. Try it at your own risk, or ask for advice from someone at ASUS or Amazon who actually KNOWS how to mount your monitors.