If you use a PlusPower on a attack that does 0 damage, will it do 10 damage? Like for example, Kangaskhan's Fetch will it do 10 damage with a PlusPower?
No, PlusPower will only add 10, *if* damage is done. Fetch doesn't do damage.
Source: WotC Chat (2000-06-01)

Relevant cards

Note: All applicable cards may not be listed below.
WOTCB1_84
WOTCB2_113
WOTCB2_113
BW1_96
DP1_109
DP3_121
HGSS2_80
PL1_112