Although PowerPoint provides a spelling dictionary which contains a list of all common words, you might be working on a particular project that uses uncommon words not found in everyday language. You could edit a custom dictionary or load a ready-made 3rd party dictionary within PowerPoint 2010 — but you can actually create your own dictionary that includes a set of words for a particular project. So, if you need to do a legal presentation for a client once a year, you could create a separate dictionary for just this one project — and then edit it as required within a text editor such as Notepad.
Learn how create and edit custom dictionaries in PowerPoint 2010 for Windows.

