You don not need to open another browser or need an internet connection, when you can have the Portal infocenter with in RAD itself.
Here is the simple step by step process to achive this.
* download the ziped version of infocenter from developer wirk.
* unzip the contents to RAD's plugin's folder ex: IBM\Rational\SDP\6.0\eclipse\plugins
* Close RAD and reopen.
Now if you open the "Help Contents" from RAD's Help menu, you will see "WebSphere Portal" in the contents.