Which side(Side 1 or Side 2) should be attached on the steel plate? And how he
attach the AD590KF to the plate?
If I want to test the temperature(about 50 degree) of the Steel Plate, then
can I place the Side 2 to the Plate, is it Ok?
Yes, you should thermally connect side 2 of the package to the steel plate,
using thermal adhesive.
You should also make sure that there is no other heat source sharing the same
GND on the PCB.