Rafi,
I should have been more specific.
First you need to create 2 User Layers:
User SilkscreenTopMount
User SilkscreenBotMount
These will also be used in your Silkscreen Generation output as shown.
You will need to Edit your Cells Pin Text Properties to look like below. Use the appropriate layers and Display Condition for the text and mount side for the part where you wish the text to be seen. In my example below, I actually have 2 Pin Text #1, one for top and one for the bottom.
I hope this helps.
Good Luck,
Vern