PIC24 are different from PIC18 devices.
There is No MSSP peripheral in PIC24, and thus no SSP... registers.
Instead PIC24 have a peripheral that is for I2C only and have I2C... register names.
Read the Datasheet for the device you are actually using.
There is No device type PIC24EP512810 ,
What device type are you actually using? There should be 2 more characters in device type specification.
If you search using Google or within Microchip website,
you will find the Microchip Webpage for the device you are using.
The product webpage for the device type have a Documentation Tab with links to correct Datasheet for the device.
Actual Datasheet may cover more than one device type.
There is also Family Reference Manual (FRM) documents that describe each peripheral type in more detail,
download and read: dsPIC/PIC24 FRM, I2Chttp://ww1.microchip.com/downloads/en/DeviceDoc/70000195g.pdf