This is hardly a comprehensive answer, but should get you started:
NSTask *task = [[NSTask alloc] init];
[task setLaunchPath:@"<whatever your path to ffmpeg is>"];
// I'm taking your word for the arguments...
[task setArguments:@[@"-f", @"concat", @"-i", @"mylist.txt", @"-c", @"copy", @"output.flv"]];
[task launch];
You may want to go further by setting up a pipe and file handle so that you can get stdout or stderr.
And yes, you'll need to include ffmpeg
in your project.