How do you figure out your Popsecret Popcorn calories?

As far as I can tell a regular bag of regular butter is about 130 calories. That's what both the side panel and MFP have suggested to me. I think I have the same brand/size as you. I would go with 150 though- better to err on the higher side, and 45 calorie difference isn't really that much.

This is what I do:
Look at the front of the box to see how many grams of product are in each bag (e.g., 6-3.0 oz. (85 g) bags)
Look at the nutrition label. The first column is for 2 Tbsp unpopped, and the serving size tells me that 2 Tbsp = 29 g.
85/29 = 2.93
So I log the whole bag as 2.93 servings.

Of course, if you're not eating the whole bag, God help you. I can't.

I weigh the bag before I pop it. then pop it. and I put mine a bowl so i weigh the empty bag and subtract that from the initial weight and that's the weight of the unpopped popcorn. so that number x/32g = number of servings per bag.

so for example total weight of unpopped bag = 100g
then you pop and empty bag and weight of bag is = 4g
weight of unpopped popcorn = 100-4 = 96
then to figure out calories = 96/32(weight of a serving of unpopped = 3 so to eat the whole bag is 3*150 or 450 calories

you could also then weigh the weight of the popped popcorn and divide into a seperate container what you were going to eat and figure out the % of the total that you were eating and figure out your "portion" of the unpopped and that's your serving.

to continue example from above:
if the weight of the popped corn is 300g and let's say you put 100g into a bowl and that's your portion you are eating 33% (roughly) of the popcorn. so you could take the total 450 calories and multiple by .33 (for 33%) and that's your calories from popcorn or about 149 calories.