Richard is spot on. The 2550 have two MSSP modules so there are two SSPCON registers.
I've found that sometimes MeLabs have aliased the SSPCON to SSPCON1 so code written for devices with one MSSP module would still work on devices with two but apparently not so on this device.
As always, when i doubt read the datasheet and by all means look at the .PBPINC files where all the registers that PBP is made aware of is being declared.

/Henrik.