I have a client who informs me that the Default HTML Buttons are not appearing with the field. I am using a mac with Safari and Firefox and they display no problem. Anyone else encountered this problem?
From the information you have provided, I can only give you the general steps that you would need to take to resolve such an issue with Ellislab.
Since this would likely be a bug, you would need to post a bug report to the bug tracker.
However, you may want to confirm this through Ellislab's official support channel before posting a bug report.
If you can't wait, then you can also modify the control panel theme. Make a copy of the problem theme first, and then modify the copy.