Here is my method - YMMV:
MiCollab Server/NuPoint Web Console/Call Flow
1. Enter The Mailbox for your main line.
2. You'll see the call flow for your auto attendant.
3. Select Actions/Recordings from the top toolbar.
4. The Recording Status screen will open with a list of menus that currently host recordings.
5. Find the entry for your main line - probably labelled "Open Menu" or "Day Menu"
6. You can Record or Import from this screen - I use Import.
Here are the specs if you are going to import a recorded greeting file - we use Goldwave to record greetings:
File Type: WAV u-Law, 8000 Hz, 64 Kbps, mono
There may be other variants of a WAV file that are permitted, but this one always works for us.