Rift Gameplay - Synergy Crystal

Solution 1:

You activate the bonuses by equipping armor (left side) that is part of the set. Set membership will be displayed on the tooltip for the armor piece.

You are wearing level 65 armor with a level 60 synergy crystal that are not from the same expansion, so none of the armor in that screenshot will contribute to T'Scain's set.

As of this writing - as well as of the time of question submission - the level cap is 70, and there are even more new sets and crystals, so everything in your screenshot is outdated. It is not worth trying to activate a synergy crystal until you are at level cap.

The general way to do so is (1) get armor from expert dungeons, raids, and/or the RIFT store (for ingame currency OR real money) that has a set and (2) buy a crystal for the set from the RIFT store (for ingame currency, usually also gated by faction reputation). Note that sometimes an armor piece needs to be upgraded to count towards a set.