GetFinalFireTime
hasn't been implemented yet.
GetNextValidTimeAfter
should be working if you're using one of the latest versions.
If you want to generate cron expressions in c# you can use CronScheduleBuilder
.
UPDATE:
You can find a simple project here (QuartzNetCronExpressions.zip) where you can see how it works.
NOTE: I've used VS 2013, Framework 4.5.1 and Quartz.net 2.2.1 (nuget package).