The following are known Action Replay Codes for Monster Rancher DS on Nintendo DS (NDS).

Game ID: CFME-C3CF3C46

Infinite Gold
021b6690 3b9ac9ff

Unlock Encyclopedia, Spell Book (Press SELECT)
94000130 fffb0000
221b668d 0000000f
d5000000 000000ff
c0000000 00000022
d8000000 021b666a
d2000000 00000000

CURRENT MONSTER CODES:

Max Fame
221b6710 00000064

Max Power
121b6720 000003e8

Max Intelligence
121b6722 000003e8

Max Skill
121b6724 000003e8

Max Speed
121b6726 000003e8

Max Defense
121b6728 000003e8

Max Life
121b672a 000003e8

IN BATTLE CODES:

Max Fame
221b6710 00000064

Max Power
121b6720 000003e8

Max Intelligence
121b6722 000003e8

Max Skill
121b6724 000003e8

Max Speed
121b6726 000003e8

Max Defense
121b6728 000003e8

Infinite HP
a22d6170 00000000
122d6170 0000270f
d2000000 00000000