Hi Itay,
You can add a correlation (in form of a covariance) between any two nodes by adding a double-headed arrow between the two nodes (e.g., in your case, the observed variables). The easiest way is to hold down the SHIFT button while you drag with the right mouse button from one variable to the other, than a new covariance arrow will appear. You have to right-click on it and set it to „free parameter“ if you want the covariance to be freely estimated from your data.
Cheers,
Timo