Fresh Jessie install yesterday.  I'm sure I told the installer to use mirrors to pull freshest pkgs.  I don't recall exactly what it did, I was in/out of the room.

And everything went fine. But now when I want to add more stuff, Synaptic can't find the internet and wants me to use the install media--which of course won't work as I used USB and it only looks for DVD.

As I dig into the issue (i've been distro hopping and also getting much more familiar with shell activities and fixes and such is Linux) I find I get more confused and it does appear that not many folks have had this issue from the lack of discussion in terms that make sense to me (at this point).

Question. 
Do i have to go in and tell Synaptic what repositories to use? 
Why? I've never had to do that before, is it something that happened in the install?
Which repos do I use?

I love the installer and most of the Devuan/xfce environment, but am really hung up now as I'm operating on a software island. Taking screenshots during install are brilliant and saved me the hassle of taking pics with my phone-already a habit.

There is no problem with network or connection for other purposes.  Streamed video without a hitch and was easier on resources than previous two installs, and I'm all for that.

As I have gotten used to running installers over and over, is that the fastest way to fix this?  But also, rather learn to fix it in the shell, so long as I understand why and how we got here.

Thanks so much.

I want to be able to replace my Mint install with something similar but different and Devuan--aside from this issue looking pretty good so far.  I'm only beginning my "official" study of programming and yet have a lot of Linuxing to learn as I've only been a user for a few years.