how to use this.symbol?

There is little need to use 'this.symbol' in AS3 though you can if you wish to.  Just using 'symbol' alone is sufficient.  'this' becomes more useful when you are using bracket notation for a string to target an object, as in...  this["symbol_string"]

Give an instance name for the object (if the object is on the stage: select it > go to properties window > type a name in Instance Name field, for example my_mc)

then you can call it in ActionScript:

my_mc.alpha = .5;