I am not a Metastock user, so I am afraid I cannot tell you exactly how to do it. However, I can tell you what you need to do in general terms.
I am aware though that Metastock does not like negative numbers, so you will have to add a large number like 50,000 or 100,000 to the beginning of your line. This will not affect its interpretation, because it is the slope of the line that is important, not the absolute value.
What you need to do is create a file from which you can draw a line chart. The line chart required is of the cumulative difference between advances and declines. You will need to take all your historical data and calculate the cumulative total of advances minus declines. Then each day you update it by taking advances minus declines and add/subtract that difference to the cumulative total.