I have a lenovo thinkstation d20 and I get a decklink SDI 4K, is already install I like know how to enable the media composer i have 8.7.2 version, for viewn in a esternal tv monitor thru hdmi i have a convertor from SDI to hdmi

The client monitor toggle is blue & is located next to the video quality switch below the timeline. Not sure if this is what you're looking for? 

Have you installed Blackmagic Designs latest Desktop Video software? Then set up this softwares output.  

If all of that is setup then Mark's  information will make sure MC is talking to the Decklink.

Once you've installed the correct software/drivers as previously mentioned, you should notice that when MC is starting, at some point in the startup sequence, the Blackmagic card is mentioned - right at the very top of the screen where it says 'Media Composer'.

If you're still having trouble, take a peek at this thread: http://community.avid.com/forums/t/132803.aspx.

There are more on point threads in the MC fora. Difficulty getting MC & DTV to work together properly comes up from time to time. Not every version of BMD's driver (desktop video) runs properly with every version of Media Composer on every computer. Sometimes finding a working combination requires more patience than one might otherwise anticipate.