there is no "dipstick", only a threaded plug on the side of the transmission, accessed from underneath. It is assumed that your trans will not leak, nor use fluid so you don't have to add. It's assumed you will have the dealer check it and fill it to the plug level then never need servicing. It is assumed you will have the dealer look at it every 80K or so. YMMV... :)
it never needs to be changed
In the right compartment of your trunk.
This is a sealed transmission. It does not have a dip stick. Read the manual. sometimes that sealed transmission leaks by way of the radiator
and what type transmission fluid
When you have the hood up and your looking at the engine, it's the yellow or red colored dipstick that's between the engine or radiator. The one in my 740i is yellow.
Carefully.
In the Trunk.
As with most BMW models the BMW 740i sedan comes equipped with a standard 8-speed automatic transmission. If one wanted to upgrade, it would be dependent on the make and model of the upgraded vehicle.
On bottom right side of trasmission
On a 98 740i, the battery is located in the trunk behind a carpeted cover on the right hand side, just behind the right rear wheel well.
Inner side of driver's side frame rail
Take to a qualified BMW mechanic.