Generally, the connection information to make this happen is stored in a file called eclipse.ini, located in the Process Designer install directory. Specifically, there is an option in that file called:
and it needs to be set to the address of your Process Center, like so:
This is the first place to check to ensure this location is correct. If it is, and the Process Designer will still not connect, it could be because of how the Process Center was installed and configured. If the Process Center was configured using only the short hostname and not a fully qualified hostname, this can cause issues. The reason is that even though the initial connection is made, subsequent exchanges fail since Process Center sends back just its short hostname and the Process Designer machine may not know what that means.
If Process Center is configured using just the short name of
To see if this could be the issue, you can check the directory where Process Designer is for a .txt file with a name like:
There is a technote from IBM WebSphere Application Server which details a similar issue, though the files and symptoms may be slightly different, the solution of editing the host's file is the same: