I use it like this in menu bar: ... <li><a href="<$MTBlogURL$>about/"<MTIfFolder name="about">class="active"</MTIfFolder>>ABOUT</a></li> .... But now in mt 4.21 , it will tell me there is an errer:outside of proper context
Well , how to use the tag now?
Reported on Movable Type 4.2

I think you need to put it all inside an <MT:Pages> .... </MT:Pages> container tag pair.
Ok,it works now. Thanks a lot! By the way, (mt:Pages) or (MTPages) - tags like this, which is recommended ?
I have the same issue. I tried your solution, Gautam, but am still geting the same error. The tag is located in an include module that is pulled into both a main index and pages; neither likes it. Any suggestions?
@dimlau. I don't think it makes a difference.