If I am not a Knight, how can I get Defend the Line (Fighter Utility)?
Human bonus at will power does not help, as it is not a 1st-level power, and not an attack power.
RAW, the only way you can get it is by Paragon Multiclassing. The entry for paragon multiclassing in the Player's Handbook (209) specifies that:
At 11th level, you can choose to replace one of your at-will powers with an at-will power from your multiclass class.
Note that unlike the Half Elf's Dilettante or Human bonus at will, this does not specify an at will with a level, or even an attack power at all. And though there is no official "Knight multiclass," Defend the Line is technically a Fighter utility.
With that said, this is an extremely expensive way of going about getting Defend the Line's feature. If we're comparing Paragon tier, a flail user with Lashing Flail and Hindering Shield can accomplish the same thing. Any one handed weapon user can accomplish it with a Lightning Weapon, Mark of Storm, and Hindering Shield. A Half Elf using Dilettante and taking the Versatile Master feat could take Eldritch Strike (potentially with Adept Dilettante to change the statistic used, if necessary) and Hindering Shield, and obviously a pure or hybrid Warlock could do the same. In Epic, marking combined with the Gladiator theme and the Keep Them Close feat can also slow enemies.
There are circumstances where Paragon Multiclassing might be worth it to pick up unleveled At-Wills, such as picking up the Scout's Dual Weapon Attack, or a Half-Elf going with Paladin through the Squire of Righteousness feat for Defender Aura and then using the second half of Versatile Master to swap in the Berserker's Vengeful Guardian power. But Defend the Line is replicated easily enough that it wouldn't be worth doing.
RAW, you can't get Defend the Line unless you're a Knight.
Defend the Line is a utility power granted by the Fighter Stance class feature of Knights. It has no level, and thus cannot even be replaced by Acolyte Power when you multiclass.
There's space for a multiclass entry feat that gives you the use of some Fighter Stances, but multiclass support is very patchwork for Essentials, and as far as I know one doesn't exist.