How can I get MC12 to display the library data when it first opens?
You can do this by CTRL + O and selecting Startup on the left pane. On the right pane, right under "Startup Interface" will be "Location: ...". Default is "Location: Start" but you can have it display your audio, video, etc... Just click to change location.
I can't find the library it created when I installed the program
Not sure I understand this. Are you talking about File > Library > Library Manager...?
or
Are you talking about the music, video, etc...Library. If so have you impoted them into MC? If you have it should show up when you click the Audio, Video, Images or Documents Icon (Document has no icon) in the upper left.
Will MC12 let me import data from an Excel spreadsheet, edit and modify the resulting library (?)
I'm not an expert but I don't think you can do this.
then rewrite the ID3v2 tags from the edited library back to the files?
Options (CTRL + O) > General > Importing and Tagging
If "Update tags when file info changes" is checked yout tags in the files will be updated when ever you make changes.