Matrix can be used cross-platform on any client which supports the protocol.
When using a software client the username and password set during the registration process need to be entered as well as the matrix server address https://3000.chat.
A username is formatted in the syntax @USERNAME:3000.chat or @USERNAME:matrix.org depending on which server the username was initially registered.
The same syntax needs to be used when connecting to other users via a client.
Alternatively to the built-in webclient on 3000.chat the public Element web client can be used as well when selecting https://3000.chat as homeserver address.
The following site lists software clients for all possible platforms.