My magnesium level is:0, 64 mmol/L (ref: 0, 75-1).I`m taking daily 250 mg magnesium, but it seems to be not enough. How much is recommended in this case?

Review with your doc. Magnesium is one of those minerals that can cause a lot of problems if you take too much of it. The recommended dietary allowance of magnesium is 320 mg; getting this from food sources is entirely safe. Taking more through supplements may cause diarrhea, nausea/vomiting. I'm not sure why you think 250 mg is not enough, but taking up to 320-350 per day should be safe. But check with your doctor:)
Wrong test. Consider testing RBC magnesium instead of serum magnesium. Serum magnesium is a poor indicator of tissue magnesium levels. You are probably correct that you need more magnesium. More information on: http://drfairchild.blogspot.com/2012/12/magnesium.html.