Is the same for 8. Once you do figure it out you never forget!

Intuitive or not, the process is quite well documented in the Acrobat complete help found under the help menu.

Taken from Acrobat 7 complete help file (should be similar in 6 and 8 help files):
Using commenting tools> Adding comments in a text box

To add a text box comment:
1.
Select the Text Box tool on the Drawing Markups toolbar.
2.
Click in the PDF document to create a default-sized text box, or drag a rectangle to define the boundaries of the text box.
3.
Use the Properties toolbar to change the color, alignment, and font attributes of the text you type, and then type the text. The text box expands horizontally to fit the text until you press Enter or Return. If the Properties toolbar is not visible, right-click (Windows) or Control-click (Mac OS) the toolbar area, and then choose Properties Bar.
4.
Do any of the following:
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Click the text box to select it. Use the Properties toolbar to change the border and fill options.
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Double-click the text box to edit the text or to change the text attributes. Drag across text to select it, and then select options from the Properties toolbar. When you're finished, you may want to close the Properties toolbar.
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To change additional properties, right-click (Windows) or Control-click (Mac OS) the text box, and then choose Properties.
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To delete the text box, right-click (Windows) or Control-click (Mac OS) the text box, and then choose Delete. To resize a text box comment, select the text box using the Hand tool or the Text Box tool, and then drag one of the corners.