Hello. How to add the second IPv4 address for the instance?
It depends on what you mean by the second IPv4 address.
If you mean the second address as on in addition to the private 10.10.10.X IPv4 address, you do that by associating a floating IP - http://wiki.dreamhost.com/DreamCompute_Getting_Started#Associate_a_Floating_IP
If by second IPv4 address you mean a second floating IP, there isn’t a way to add more than one floating IP to an instance unless it is connected to more than one network. Right now we only support one private network but plan to add support for multiple networks in the future.