The key property changes for this menu sample are only made within the main menu and are as follows:

Set the left to 0 or delete the left property
set menuwidth="100%"
set menualign to either "left, center or right"

Note the addition of quotes around "100%", this is needed because the value needs to be passed as text.

your menu should look similar to this:

with(milonic=new menuname("Main Menu")){
style=menuStyle;
top=220;
menuwidth="100%";
menualign="center";
alwaysvisible=1;
orientation="horizontal";

More positioning possibilities can be achieved with the following samples Attached to a Table Cell and Positioned with Offsets