Select the "Arial Unicode MS" font and then select "Insert" | "Symbol" item
while you are editing the text, locate and select "Mathematical Operators"
in the Subset box, browse through the mathematical symbols for the therefore
symbol.
I've never heard of the 'therefore' symbol before... So a quick search on
google reveals
it's probably three dots in a triangle layout.
If that's the case
in powerpoint, while you are editing a textbox, choose "insert > symbol"
change the current font to "symbol" and choose character 92 from the symbol
font.