德 dé is traditionally translated as "virtue," though in the practical, not the moral sense, as in Moliere's joke about opium's putting people to sleep through its "dormative virtue" (i.e. sleep-inducing power) or the simple phrase "by virtue of." To avoid the misreading as a moral term, I am translating it "powers."
Centuries before Zhuangzi, it was used primarily to describe the charismatic authority of an effective ruler, something that was respected but mysterious and so the subject of much discussion. As a political discussion, the default preliminary answers were what you might expect: military force and the threat of violence. Duke Huan, with his minister, Guan Zhong, realized the power of money and focussed on economic policy with great success. Confucius emphasized the role of gratitude in motivating allegiance and advocated kindness as the way to lead.
Over time, it was acknowledged that different things had different powers; that is, different ways of influencing the world around them. For better or for worse, attractiveness, for instance, was described as 女德 nǚdé, "woman's power" (Zuo Zhuan, Duke Xi, 24th year). Powers need not be good: the duke's heir in 4:04 is described as having "naturally murderous powers." They can also take on various forms: Whish and Whoosh in 7:07 attribute their sense of to the powers of their generous host (which I translate liberally as "powerful favors" to get point across). I prefer "powers" to "power" to prefer this diversity and unpredictability.
Zhuangzi broadened the question beyond politics, from what makes kings and states dominant to what makes things work in general. The powers of the spiritual people in distant Maiden Mountain in 1:03, for instance, "keep things from harm and ripen the harvests." Given the diversity of powers out there, it is less a question of coming up with the one true power than of harmonizing existing ones (5:01, 5:04). The paradigm for this is "nature" or "heaven."