When you choose use existing disk you have to choose the vmdk file which is acting as the pointer of the RDM. Just like you would add any existing disk to a virtual machine.
if you cant see the datastore of file then it sounds like the 2 esx hosts are not connected to the same storage at least not to the storage where the virtual machines are installed on,
When setting up the Quorum you would have selected rdm and then pointed it to a LUN, the it would have put a <machine name>.vmdk file in the virtual machine location (a datastore presented to ESXi host) it will display the size of the RAW LUN. this is the VMDK file you must select on the second virtual machine.
you dont require vCenter I wouldn't think as there is nothing in setting up a MSCS cluster that requires anything from vCenter. Obviously if your sharing storage it is neater to use vCenter.