First, make sure you click the following link: http://world201.runescape.com (you need to make sure you are on world 200 for it to work)
1. Copy %USERPROFILE%\jagexcache\jagexlauncher\runescape into the address bar of My Computer
2. Open "runescape.prm" inside Notepad
3. At the end of the line ”-Dcom.jagex.config=http://www.runescape.com/k=3/l=$(Language:0)/jav_config.ws” add ?f=201 so the line should look like: -Dcom.jagex.config=http://www.runescape.com/k=3/l=$(Language:0)/jav_config.ws?f=201
4. Save and launch the client.
And to Launch the Beta in a Client for a Mac please complete the following steps:
1. Go to the applications folder and find RuneScape.
2. Right click and select ‘Show package contents’
3. Open the folder ‘Contents’
4. Locate and open the file info.plist
5. After the .ws in the line ”http://www.runescape.../l=$(Language:0)/jav_config.ws” add ?f=201 so the line should look like: http://www.runescape.../l=$(Language:0)/jav_config.ws?f=200
6. Save and launch the client.
PLEASE NOTE: That while the ?f=201 is set the downloadable client will only be able to access the beta. To access the normal game client again you will need to remove the ?f=201.
EDIT BY MOD HODGE: If anyone is still experiencing issue with connecting their client to the Beta, try changing the line; -Dcom.jagex.config=http://www.runescape.com/k=3/l=$(Language:0)/jav_config.ws
Red are commands or locations, and green is to add to that line. I have tested this myself and it does work, much more open when it's on the client and not on your browser (and probably runs better for most people). I had to do both changes (the ?f=201 and the world201 edit on the url) for mine to work, but I'm on Vista, so that may affect it. Also to any Vista users, the files you're looking for will be located in your C: drive, so check there first.
Also as a simple replacement, make a copy of the original and name it like runescape2.prm so when you're done, delete the one you altered and rename that one back to runescape.prm (if you want to return to normal gameplay).
Hopefully this helps in you enjoying the Beta more, so give it a shot and have fun!
Edit: Thanks to Kairi (or Seinami if you prefer ), she has made a video tutorial for doing this too if all that seems very confusing .
Edited by St Squiggy, 18 July 2012 - 09:44 PM.