Critical Warning: Timing requirements for slow timing model timing analysis were not

It sounds like you are new to Quartus. For information about the Quartus timing analyzer, see the chapter for the analyzer you are using (Classic Timing Analyzer or TimeQuest) in the Quartus handbook, Volume 3, Section II. The handbook is available at http://www.altera.com/literature/lit-qts.jsp. (The handbook on the web site is for version 8.0. If you are using a recent pre-8.0 Quartus version, most of the information in the 8.0 handbook will apply.)

After you have the design constrained correctly including any false paths (mentioned by Mark) and multicycles, use "Tools --> Advisors --> Timing Optimization Advisor" in Quartus to get suggestions for fixing timing violations.