To import this BBCode, log into your AdminCP, navigage to the "Look & Feel" page, click on "BBCode Management" in the sidebar, then scroll all the way down until you see the Import New BBCodes box.
Extract the included "youtube_subscribe_bbcode.xml" file and import it here.
To use this BBCode, just wrap the name of the channel you wish to generate a Subscribe button for in "[ytsub]" tags. For example,
That's all there is to it, enjoy!