The multihop network is compatible with any OpenVPN client, including Viscosity, Tunnelblick, OpenVPN Connect for iOS/Android etc.
To use multihop you need to modify the username you enter when connecting. You simply need to append an '@' symbol and a location code and your traffic will be routed through the specified exit location (regardless of which server you connect to).
For example, if you connect to the Romania server (ro.gw.ivpn.net) and wish to exit your traffic in Switzerland then you would append @ch to your username e.g. ivpn12345@ch - that's it! If you connect to France with the same username then the traffic will still exit in Switzerland.
To lookup the location codes, open the server status page. Note the server names are in the format XY.gw.ivpn.net where X is the location code and Y is a number uniquely identifying the server. Ignore the number (the entry server will automatically connect to the least loaded exit server) and use the code before the number. Below are some examples: