Here's a checklist of things that might have gone wrong. I know you mentioned some of these already.
1) Find a patch of muddy ground for the farm. This may involve the use of water.
2) Define a plot (b -> p), adjust its size (u/m/h/k) and place it (enter).
3) Ensure that you have some farmers with the Farming (fields) labor enabled.
4) Query the farm (q) and select a crop for each of the four seasons (look at the query screen for how to do this). Newly built plots will be fallow and never be planted until you select a crop.
5) Step 4 is different for above-ground and below-ground farms. Dwarvish crops like plump helmets will only grow below ground while prickle berries, rope reeds and other mysterious alien crops only grow above ground.
6) Make sure that you have a stock of seeds for your chosen crops. Plump helmet spawns come from eating uncooked plump helmets or traders (that's purchased from traders, not from eating traders), but above-ground crops can usually be gathered with the plant gathering labor.
7) Your farmers need to be able to get from where they are to the seeds and then back to the farm. Locked doors, burrow restrictions, floods and other inconveniences can prevent them from trying.
8 ) Nothing will grow in freezing weather, underwater or in magma. Don't even try to make an above ground farm on the side of a glacier.
If all of these conditions are met then your dwarves should eventually get around to farming. Just remind them where dwarven wine comes from if they get lazy.