Note that you can find topics by searching the forum - the link for the Search is located at the top of the page. I found the above post by searching for the for the word "angstrom" in the entire message.
If you are using Origin 7.5, this is covered in the Origin Help file topic Text and Drawing Objects >> Text Objects >> Creating and Editing Text Labels, under the subheading To add special characters to your text labels.
In this particular case, note that the Angstrom character is not part of the Symbol font set, so you should choose another (e.g. Arial) from the Font drop-down list in the Symbol Map dialog box.