I haven't tried D2SE's "Straight to BNet feature" but it sounds like you should keep things simple for now, at least until you know you've got the mod up and running. Try just making a single player character.
Once you know you can make characters, then we know you've had the mod correctly installed all this time. Go back to the main menu, and under the Battle.Net button, select the "Arimyth Network" gateway (rather than USWest or USEast - these are Blizzard's servers)
If you don't see Arimyth Network, then in the HU package you downloaded (I'm guessing v1.6c?), there should be a file "hu.reg". Double click it and accept that it will write information to your registry. Reopen D2 and let us know if you can see Arimyth Network in your Gateway list.
If this doesn't help, then we might need more details about what you're doing.
- What version (HUv1.6c or v1.7)?
- D2SE expects mod files to be within ".../DiabloII/MODS/<mod folder>/", are yours there?
- Are you installing into C:/ProgramFiles/? This can cause Administrator-privilege issues for Windows.
- From D2SE's menu, are you selecting (or double-clicking) HU's picture in the left half, and then pressing "start plugin" button at top right? (The "Start D2" button ignores the MODS folder, I think)
Free free to add extra install details, we can skip them if they're irrelevant