Make sure memory module meet Specification. If you are not sure, it is recommended to use a memory module listed in the Hardware Compatibility List. If you need to resort to memory outside the list, make sure you run the memory test twice in succession before trusting the memory.
The ReadyNAS NV/NV+ utilizes one 200-pin DDR SO-DIMM module. Installation instructions are posted here. Also, see this user contributed YouTube video walk-through of the process.