sudo pacman -S openrct2
Alternatively, if you want the latest development build, install the openrct2-git package from the AUR. The development builds are analogous to beta versions - they should work, but don't expect everything to be perfect.
When OpenRCT2 first launches, you will be prompted to select the directory where you installed RollerCoaster Tycoon 2. You can also set the RCT2 path from the command line should you wish - use the command below.
openrct2 set-rct2 /path/to/rct2-install
This concludes the Quickstart Guide. It's time to play OpenRCT2. If you have any questions, don't hesitate to ask for help on the forums.