1. Edit the page
2. Add a Content Editor Web Part
3. Edit the Contend Editor web part properties, remove the chrome, and make any other changes as desired, rename the web part to something more meaningful than 'Content Editor,' (for example, 'Pop Up'), and then [Apply] and [OK]
4. Select Click here to add new comment at the Content Editor Web Part
5. Locate the HTML icon within the Markup section of the ribbon, select it, and then select Edit HTML Source
6. Type in or copy and paste the code below in the HTML Source web page dialog
<alert("Alert Message Here")
Note: Replace the Alert Message Here text with the appropriate message that you wish to convey.