You're probably in an area which is difficult, but not impossible, for the various civs to access. The mechanics of this are not well-understood, but it appears that great distances between you and a civ, difficult terrain (glaciers, deserts, and thin mountain passes, for example), and the like can delay caravans and invaders significantly and make their approach intermittent.
Civilizations will appear on the c screen only after they have actually come to your map. If there is a goblins civilization at all that is not totally blocked off from you, you will get thieves/ambushes/sieges eventually. However, again, it may be that the route to you is difficult, which would slow things.
To grow dimple cups, you need dimple cup spawn. You can embark with them, but they tend to quickly get used up initially to grow the first crop of dimple cups. You then must process the dimple cups to get more spawn; this can only be done by a miller at a millstone or quern. Dimple cups cannot be eaten (at all), cannot be processed into anything but dimple dye, and thus often are never used. They are in fact practically worthless if you don't have a cloth industry, though buying up large quantities of undyed cloth from traders and dying it can be a profitable enterprise in itself.