Hi, skc! And welcome to the insanity.
Looks like Wardog beat me to the punch, but I'll add to it anyway.
To answer your 1st Question:
Joining online may be accomplished two ways. A) Through Hyperlobby (requires that the Host has started a server within HL). There you can join a mission in progress, or join a Coop Room and wait for the Host to start the thing, and B) When joining via IP Address, in the "Multiplay" screen of IL-2 (the game) click the "Join Server" button and enter the IP Address of the server. The IP addie is normally posted by Genosse for all the Sunday Sessions. You are also cordially invited to join the Hysterical Encounters sessions every Friday at 1900 hours (7:00pm/GMT-6) Mountain Time, and we'll provide the IP addie via Comms.
Response to 2nd Question:
Some of us have the HSFX v4.11, some of us have the Ultrapack. So far I don't recall any real issues between the two on Sundays. Fridays the HE sessions are flown with HSFX v4.11 only, to ensure compatability. Mostly a personal preference at this time.
That said, you most likely will notice
some anomolies regardless which method you use. It's all a part of the "game" of Mods.
My current setup includes seperate installations of IL-2 v4.09m and an HSFX v4.11 mod.
With any type (HSFX or Ultrapack) go to your game folder in explorer, look for the "jsgme.exe" file and send to desktop. It creates an icon for the JSGME utility. Open it.
Within the utility GUI will be a list in the Left-hand pane of all mods included with your version of HSFX or Ultrapack. Click any one of them and move it to the Right-hand pane to enable it. Alternatively you can move all at once by clicking the ">>" button.
You can disable any one, or all, using the same method to move the mods to the Right-hand panel.
It's important to show up a bit early to any session to establish which additional mods, if any, the host will be using. This helps ensure a timely start for missions (right

) and allows for an opportunity to troubleshoot and hopefully correct any improper installs that may prevent you from joining.
Hope this helps.
Helo