Since it might be unclear to some. The client is setup to connect to a localhost environment. Once the server repo is available, you'll likely use the localhost environment to do most of your testing. Since it isn't available, you need to connect to a server already running, which will be core in this case.
The client has some built in servers to select from. You can see which by looking in src/kabam/rotmg/build/impl/BuildEnvironments.as. The selections of interest here is NILLYSREALM and NILLYSREALMTEST with the values "nr" and "nrtest" respectively. You may use these string values in the ENV flashvar.
In WebMain.as you'll see this bit of code:
As you can see, the ENV variable is set via the flash variable "env". This is used to dynamically set the environment so that we can use one client rather than multiple clients to test with. If the env flashvar isn't present, it defaults to "localhost" which will attempt to connect to the locally running server.
Basically, to get the client to connect to the core server all you need to do is uncomment that highlighted line and replace the value with either "nr" or "nrtest". "nr" will give you an environment that closely matches what production gives you. "nrtest" will give you access to additional debug options.
If you want to learn how to setup your own environments, check out this commit I made. It includes all the necessary edits needed to add an environment. https://github.com/cp-nilly/NR-27.7.X13/commit/9f58f5047ddf19bf4c635a1fc45316a2cc42fd44