While the link isn't available on the download page, you do have the option to go here:
http://forum.us.runesofmagic.com/showthread.php?t=66641
That link is a list of individual installation files for both clients, Full and Slim. I don't know what your nominal download speed is, but you can always try these files. I'd recommend downloading no more than two files at a time, and do not mix and match the files from the two lists. When you're finished, start the game installation with the RIMSetup.exe file.
Be aware that there are probably a dozen updates that will have to be downloaded and installed
after you've installed the game and started the game launcher for the first time. That can take approximately 1.5 hours, give or take, with a nominal 10Mbps broadband download. Longer, obviously, with a slower connection.