I know this may seem like a stupid question coming from a gym leader, but 
does plus power double when you opponent is weak to your active pokemon?

ie squirtle with a plus power attached using bubble against charmader.

The reason i ask is because of something i read in the extras section of the
starter CD-ROM.  the section in question is from the article entitled
"starting from scratch double slap and poison stinger" by michael mikaelian,
and it goes ". If the defending Pokémon has a weakness against your Pokémon,
Plus Power does 20 damage to it instead of 10." as i asked before is this
correct?

This is a pretty good questions.  We'll take a second to review the order in which things are done and checked for.

1) Check for and apply weakness
2) Check for and apply resistance
3) Check for and apply trainers (example: PlusPower & Defender)
4) Check for and apply Pokemon Powers (example: Invisible Wall)

So, to answer your question, PlusPower is applied after weakness is, so that PlusPower on that Squirtle will deal 30 to that Charmander, not 40.

Scott