Hello @Decry1337. My apologies for the late response.
It seems you do not have the Mega Bracelet item. Therefore, you are unable to receive any Mega stones. In order to get the Mega Bracelet, you must complete a short quest in Vermilion City Library. Here is a guide on the Mega Bracelet quest, if you should need it.
Unfortunately, the Easter event is over, so you are now unable to obtain the Kangaskhanite if you did not complete the quest. You will have to wait until next years Easter event to complete the quest and claim your prize.
However, if you did complete the quest, you can receive the Kangaskhanite from Assistant Theodore after completing the Mega Bracelet quest.
I hope this helps you and if you have further questions, please let me know.