ASUS Eee PC 1015PEM
Follow the guidelines for the 1005P
The wifi card is a broadcom 43xx series so the appropriate modules should already be included: follow the instructions on the Broadcom wireless page. It's likely that the page will recommend the brcm80211 module, but keep in mind that this is not available on the installation media (because the kernel version is too low). During the installation, you should instead use the b43 module, which is included with the instalation media.
If you choose to continue using the b43 module after installation, you can find it on the AUR atAUR.