How to make microwaved condensed milk caramel sauce properly?
Put Sweetened Condensed Milk into a microwave safe bowl and whisk it thoroughly until its smooth and the thicker stickiness from the bottom of the can that has settled is evenly distributed again.
Place in microwave at 50% power for 2 minutes then take it out and stir thoroughly. Do NOT use full power!  If you do it will burn, and it will look similar to what you mentioned was happening to yours - it turns into a brown dry solid.
Put back in microwave (50% power again) for 1 minute increments, stirring in between each additional minute until it has reached the level of darkness you are looking for. 


Hope this is what you are asking for.