Only the indexed addressing is allowed: Imm(Rn).
The address is computed by adding the immediate value (Inm) to the value read from the register (Rn) . Memory accesses are always computed as complete words, i.e. the immediate value is interpreted as a word value (and not as a byte value).
Example
// This example access to memory word 5 (3 + 2) to store R1 contents
ADDI R4 R0 #2
SI R1 3(R4)