The salt substitutes are called No-Salt or Half-Salt and are in most big grocery stores. Atkins says to use these on your food and it should be enough.
If you choose to get potassium supplements, follow the dosage instructions on the package (3 x 99 mg per day. No more.)

PEOPLE WHO TAKE ACE INHIBITORS FOR HIGH BLOOD PRESSURE MUST NOT TAKE POTASSIUM IN ANY FORM!

You could also take some calcium and magnesium (ofter combined in one tablet) for leg cramps. Or just eat 5 grams more carbs - stick to veggies - for a day or so for the fatigued and aching muscles.

You'll feel better soon - stick out the first week and things improve.