This will all change with OpenOffice.org 3.0, which is bringing in Microsoft Office-style display of notes. However, until then, here's some good stuff to know.
You can insert a note, aka a comment, by clicking in the text and choosing Insert > Note. You get this box; you type; you click OK.
It's not big and not obvious. One thing you might consider doing is to choose Tools > Options > Openoffice.org > Appearance and choose a different color for your notes.
It doesn't make it bigger but if the color is more obvious to you, then that's good.
Here's something else to know. The note display is affected by whether nonprinting characters are on. Choose View > Nonprinting Characters to toggle back and forth.
Now, as you can see the same two notes are shown regardless of whether nonprinting characters are showing, at this point. The document is in .odt format, normal format. BUT if you save the document as Word format, AND someone adds comments/notes using Word, AND then you open up the document again in OpenOffice, you won't see the notes they added.
UNLESS you have nonprinting characters turned ON.
Here I am opening up the document again, the .doc format document, in Writer. Nonprinting characters are OFF so you don't see the comment that was added in Word.
Choose View > Nonprinting Characters, and it appears. Look at the end of the paragraph.
Here's another way to more easily see all your comments, without having to look for the little yellow or green or magenta marker. Press F5 to open the Navigator, scroll down, and find the Notes item. Click the + to expand it and you'll see all your comments. Double-click one to go to the place where the comment was inserted. See that even though nonprinting characters aren't showing, all three comments are shown in the Navigator.