With the memory location aquired. As in here.
How can I write on it?
Without a reference to the variable created at this location you can't. No buffer overflow for you ;)
Generally in most programming languages, variables point to values stored at the memory location.
So, if you want to write to a specific memory location, you have to figure out which variable is pointing to that location and change that variable.
In terms of Flex and MXML components, the ID of the component is also the variable name.
I'm not sure the knowledge of a memory location has any practical use, though.