"In Version 4, I used to be able to increase or decrease the number of overall Newsbin connections right there in the interface. Now it seems hidden away - what's up?"
Version 5 is designed to have more of a "set it and forget it" approach. There was a need for many users to micro-manage the older version, tweaking the total number of connections so they could perform tasks like grabbing headers without running out of connections. Now, in Version 5, all the tasks that you would want Newsbin to do that require a connection to a news server are managed with the server connection settings in mind. If you want to read a post, just select it and hit Ctl-R - as soon as Newsbin can find a free connection(in most cases as soon as the next chunk is finished downloading), that task is performed. You will always get feedback in the Download tab as to what tasks are currently queued or running.
When you set your servers up(right mouse button over a server in the Servers tab, select Properties), be sure to set the maximum connections there...and then forget about it!