Thanks for using xAct.
There is no standard way to cite the xAct packages. I'd recommend to mention the names of the packages you have used (particularly if they are application packages, and not the core ones) and the website
xact.es, though I know it's still down. GoDaddy is giving me a hard time to change some parameters of the domain after some updates they implemented on August 6th. I hope to have it back online very soon.
Jose.