What do I do with this spice mix?
"Mix with...required salt" is just a kind of awkward way of saying to mix in salt to taste. Might make more sense if you read it as "the salt required to make it taste how you want".

Curd is basically a thick yogurt.

As for how to use it... I certainly might try their recipe and see if you like it (whether it's canonical chicken tikka masala or not).

But if you want to a variation of your existing recipe, just omit all the spices from that recipe and use this spice mix instead, probably starting with the same quantity as the total ground spices from the original recipe. Since the spice mix already contains salt, I might also omit the salt from your recipe and then add salt to taste once you've added enough spice.