When you call REXIS_SysInit to initialize REXIS, you supply the starting address and the size of the free memory space for REXIS’ memory allocator to use. You can declare a global byte array in your main program and use that as the memory pool:
#define MEMORY_POOL_SIZE (1024*30) // 30K bytes
unsigned char memory_pool[MEMORY_POOL_SIZE];
You can also use REXIS_MemInit to add additional free space blocks to the system.