Hello @NWM
The RFA C++ Marketfeed interface has only been released as 32-bit, please see the API Compatibility Matrix. If you require 64-bit support you will need to migrate to OMM. (e.g. using EMA or RFA).
The most recent releases of RFA C++ are available here. The RFA C++ versions that support Marketfeed are 7.6x, the later releases of RFA (since version 8.0x) are OMM only.