Essentials is a Bukkit plugin, which is for the custom-written Minecraft server called CraftBukkit. FTB is a modification of the normal Minecraft server. So they're completely different pieces of software. FTB doesn't have the custom code necessary to load and understand Bukkit plugins, and CraftBukkit isn't compatible with the type of mods FTB uses.
However! You're not the first person to want the best of both worlds and want to use both Forge mods and Bukkit plugins. Enter MCPC+, which is a further modification of the CraftBukkit server to add Forge, and can therefore accept both Forge mods and Bukkit plugsins.
Alternatively, if you are not interested in the huge constellation of Bukkit plugins and really just want the functionality of Essentials, you can take a look at a Forge mod dedicated to bringing the features of the Essentials plugin to Forge as a native Forge mod: ForgeEssentials.
The advantage to using MCPC+ is that it has native support for both Bukkit plugins and Forge mods. It's also fairly mature, so it's going to be more-or-less reliable (as much as anything is in the world of Minecraft modding). The disadvantage is that every time either CraftBukkit or Forge makes a major release, MCPC+ has to be updated as well so that it is compatible with new plugins/mods, and that takes time. For example, right now it's still on Minecraft 1.5, though for the current version of FTB Unleashed that's sufficiently up-to-date.
The advantage to using ForgeEssentials is that it is a native Forge mod, so it is simpler to install than dealing with two modding flavours and it will more quickly track updates to Forge and not stay out of date as long as MCPC+ may potentially do. The disadvantage is that it only adds Essentials-like functionality and doesn't give you access to the wider world of Bukkit plugins (though I hear that's a long-term goal). It's also still under heavy development, so it's not the complete, awesome, bug-free package that the Bukkit Essentials plugin is, but it might already offer the specific features that you're interested in.
Due to ForgeEssentials' WIP status you will probably want to go for the bigger-but-better MCPC+, but it's good to have options.