Select the cell where you want the bullet point
Press Alt+7 on the numeric keypad
Press Alt+0149 on the numeric keypad
Copy and paste a bullet character like • into the cell
Use Insert > Symbol and choose a bullet character
Type a bullet in another app and paste it into Excel
Use a formula like =CHAR(149) to display a bullet
Turn on Wrap Text if you want multiple bullet points in one cell
Press Alt+Enter to start a new line for another bullet in the same cell