From QGIS 1.8 and onwards the Plugin Installer plugin will no longer include the option to add the 3rd party repositories. This was by design and intended to move people over to using the official plugin repository at http://plugins.qgis.org/ so we can provide a richer experience and keep everything in one place.
If you have plugins that are still not on the official repository then I would strongly recommend that you migrate them as a lot of new 1.8 users will be missing out on your great work.
Nathan Woodrow 
[…] QGIS 軟體的一大優點就是會有很多第三方(3ir Party)開發出一些好用的小工具，大大擴充QGIS的功能與應用領域，不過在QGIS 1.8版中，不在預設可以載入其他第三方所提供的擴充元件，強迫大家盡量使用官方所提供的擴充元件。不過，依然有許多非官方的擴充元件很好用，例如之前介紹的Openlayer Plugin，這時就用手動方式加入非官方主機。 […]