Just pull the motor out until the belt is taught, but mot tight. If it is too tight, you will start hearing noise caused by the belt putting a horizontal force on the pulley/motor axle. The motor will be a little farther out if you use the black belt versus the clear one. (but see below)
The power cord.
Fist, you will need a cord with an IEC with a square body, such as the Furuteck 15A (which I use) or the Schurter. Next, you must adjust the height of the table, motor, and pulley so that there is enough clearance between the IEC and the underneath of the plinth (and the pulley is at the correct height with respect to the platter) Next, Make sure that the motor is close enough to the platter that the IEC doesn't contact the back left foot of the table (I suggest you use the clear belt). The power cord must be flexible and light enough that it doesn't pull on the motor or transfer vibration to the stand.
Don't worry. Once you have i right, you won't have to do it again :-)