Are you saying that you are having trouble installing a forum plugin? If so, are you sure that it is compatible with your current version of wordpress? We would need some more information to be able to help you like any errors that you are receiving and what version of wordpress you are using or even the forum plugin you are trying to install.
If you have a specific query with installation of a plugin, the first place to ask is the plugin authors site. The question you have asked here is too wide ranging and vague for anybody to answer successfully, please state what plugin you are trying to install, what you have tried already, and what happened when you tried this.