Soviet Strategic reserves are kept in the strategic reserve box until released - they don't come from the East. There are units in the Soviet set-up for the units that are deployed in the Far East. Once released they are strategically moved via rail (for free) - so they don't march on from the East edge - you need to rail them on via strategic movement.
They are released on the non-combat move phase of the Autumn 1941 turn if playing the European theater.
If playing the global game, the Japanese may attack the Soviet Union and/or Mongolia whenever they wish. If the Japanese attack either Soviet territory or Mongolia, the Soviet Far East Strategic Reserve is released and may redeploy strategically (for free – i.e. no cost to Soviet strategic rail points) ) on the non-combat phase to either the European or Asian theater. If the Japanese do not attack the Soviet Union, the strategic reserve is still released on the non-combat phase of the Autumn 1941 turn.