According to the manual, you key:
100 [x] 10 [SHIFT] [%] [+]
where [%] is on the open-round-bracket key on your calculator
Likewise, to subtract 10% instead of adding, use [-] instead of [+] eg:
100 [x] 10 [SHIFT] [%] [-]
edit: works ok on Casio fx-82MS where [%] is on the [=] key