global clock lines

You will get that message for a ripple or gated clock regardless of whether it is global, but in most cases you should make it global as you are wanting to do. See my comments at http://www.alteraforum.com/forum/showthread.php?p=1273#post1273. You won't get the warning if you create a clock setting and assign it to the ripple clock to tell Quartus the ripple clock is on purpose, but that doesn't help with the skew concerns for synchronous paths going to or from the ripple clock domain. (The global takes care of skew for paths with both source and destination register inside the ripple clock domain.)

For the "Global Signal" assignment, try the frequency_change:U1|clk_2 node name shown in the message. That's probably the same name that is listed for this clock in the Fitter compilation report at "Resource Section --> Control Signals". In general, the node name you should use in the Assignment Editor is the form of the name you see in the compilation report.

Also consider using a clock enable instead of a ripple clock as I mentioned in the post I referenced above.