1)
What was their desire?
1.
Rashi: "Mi Ya'achilenu Basar" (Bamidbar 11:4). Malbim - this was the opposite of Hashem's intent, that they not lust.
2)
How did they test Hashem in a desolate place?
1.
Radak: They requested meat in a Midbar. "Ha'Gam Lechem Yuchal Tes" (78:20) refers to meat, like "Es Korbani Lachmi" (Bamidbar 28:2).
2.
Malbim: They thought that He cannot supply all three (water, bread (manna) and meat); they doubted His ability.
