I bought acre p221w monitor recently and now I

You must ensure that you have the setting for 3D available in the monitor's settings. If so, just find where that setting resides, and implement the 3D setting for your monitor. The monitor comes with a pretty extensive manual and if you don't have it, then go to Acer's web site and find the manual, by using the monitor's model number as your search key. Once you've found the monitor, download it - it is usually in PDF format - and then look through the manual, using the monitor's model as one search key, and find the section for 3D. Then go to the settings section, and follow to the letter what it says about how to set the 3D on that monitor and, as Michael Jordan says, "Just Do It!" You can do it easily and get it done the right way.
Enjoy!