Most people wait for all fermentation activity to finish before adding the dry hops. If the beer is still fermenting, it will tend to drive off some of the aromatics from the dry hops.
Kit instructions are created for the beginner to get a beer in the bottle as quick as possible. Often, they are not the best instructions for making the best beer, and taking a little more time can help considerably.
Don't worry you will still have good beer.
You didn't do anything wrong by not taking a gravity reading. However, gravity readings are the only real indication of what is happening to the beer. For beginners, gravity readings help with understanding what is going on, but many experienced brewers don't bother with them until it is time to package the beer. When you take the sample, drink it (it is beer). Drinking it helps with understanding how the beer is doing. If you return it to the fermenter, you risk contaminating the batch.