CONFIG_MEMORY_HOTPLUG is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT

Memory hotplug

modulename: hotplug-memory.ko

and/or

memory.ko

configname: CONFIG_MEMORY_HOTPLUG

Linux Kernel Configuration
└─>Memory Management options
└─>Memory hotplug
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
source code:
/arch/powerpc/platforms/pseries/