You should be able to do this if you have OS X 10.3 or later and PowerPoint
X or later.
Begin by launching PowerPoint, and going to Tools > Customize > Customize
Toolbars/Menus (or Tools > Customize in PowerPoint X). Click the "Toolbars"
tab, and make sure that "Menu Bar" is checked in the list. Note that the
top toolbar allows you to edit the contents of the menu bar. Now, click the
"Commands" tab. In the left pane, select "Macros". From the right pane,
drag the macro for which you want to create a keyboard shortcut onto the
"Tools" menu on the top toolbar without releasing, and drop it into the
"Macro" submenu. That will place the macro in the menu. You can access it
by going to Tools > Macro > <your macro>. Repeat for as many macros as you
wish.
To create the keyboard shortcut, launch System Preferences, and then open
"Keyboard & Mouse Preferences". Click the "Keyboard Shortcuts" tab, and
click the "+" button to add a new shortcut. Select PowerPoint as the
application, and for the menu item, type in the exact name of the macro.
Create the keyboard shortcut, and click OK. Repeat for as many macros as
you wish. You'll need to quit & relaunch PowerPoint for these keyboard
shortcuts to become available.