1. Go to http://www.wolframalpha.com/addtoyoursite.html, select the widget size and copy the code into your clipboard
2. In LibGuides, create a Rich Text Box, then click on "add text" inside of it.
3. Switch to the plain text editor (the link at the top of the rich text editor popup) - this is an important step.
4. Paste the code from step 1), i.e. from your clipboard
5. Click on "Save Changes"
6. Refresh your LibGuides page by clicking on Refresh button in your browser.

That should do it. Enjoy.