I would go ahead and get the pH up first--hopefully it's only 6.8 and the first addition will get it up above 7.0, then you can go on with your shock. If it's below that, getting it up is gonna be more critical to preserving your liner.

Have you backwashed your filter since you added the stabilizer through the skimmer? If so, then you've backwashed it out and that's why it's not showing up. You'll need to add more, but don't backwash for at least a week after the addition. If not, then just give it 3-4 more days before you try to test for it, because it does take a long time to dissolve. AS for the stabilizer in the bottom of the pool, make sure that you're brushing it around on a regular basis so it doesn't sit on your liner and stain it.

Janet