Select:

Data>Validation

from the drop down choose "List"

then in the textbox below enter the value's you want seperated by a comma like so

100,200,300.

or if you know how create a list of the numbers you want to use then highlight them and in the white box at the top left of the screen type a name and hit enter.

then in repeat the data validation part but in the box type:

=Thenameofyourlist

hope that helps