Draw|Autoshapes. Choose Lines and select the Arrow option. Click and drag on
your slide to create a basic arrow.
It will still be selected (or just click it on the slide to select it).
Choose the line color icon on the drawing tool menu to change the color.
Choose Line Style to change the thickness of the arrow. Choose Arrow Style
icon to change the direction of the arrow.
Alternatively, right-click the arrow -- or even a plain line -- on the slide
and choose Format AutoShape. On the Colors and Lines tab you can change the
beginning and ending styles and sizes for the arrowhead and also the
color/thickness/etc.