The precompiled client found in Rotmg123.5.1/out/RealmClient named WebMain.swf is set to ip 127.0.0.1 port 8080 by default.

In order to get the client source to run you need a ActionScript3 Editing software with a compiler such as IntelliJ or Eclipse.
You'll also need the SDK which is Flex SDK 4.6.0 (http://download.macromedia.com/pub/flex/sdk/flex_sdk_4.6.zip)

As of this release I will not give any further instruction or help on using the AS3 Client Source.

I strongly recommend using the Attached Server Source with the client as it has been modified to run the client "out of the box".
It is the same Base Source Code as the one used for Seraphs' Dominion. It uses a MySQL Database.
(Look at the .cfg files in bin/Debug/ located in the Server Source Folder)