Psionics in the Realms
The Forgotten Realms Campaign Setting presents Faerûn, (and all of Toril), as a non-psionic world. That does not mean, however, that you cannot use the Psionics Handbook/Expanded Psionics rules for your Faerûnian campaign. Psionic powers are rare, and most people find little difference between psionics and magic. Do not use the Nonmagical Psionics variant described on page 80 of the Dungeon Master's Guide in your Faerûnian campaign.
Reactions to Psionic Characters
Psionics are rare on Faerûn and largely unknown except in areas where psionic monsters live. Such areas include the South, where the yuan-ti have influence (particularly the city of Hlondeth on the Vilhon Reach) and the Underdark (thanks to the influence of the mind flayers). Waterdeep undoubtedly holds a few characters with psionic abilities, as do large urban areas such as Amn and Calimshan.
In general, psionic characters can expect the same reactions and attitudes that arcane spellcasters encounter. Areas that welcome or at least accept wizards will welcome psionic characters with equal enthusiasm (or lack thereof). In areas where wizards are feared or shunned, such as Amn, psionic characters will have to tread carefully.
Many spellcasters are likely to view psionic characters as potential magical rivals, so psionic characters might consider concealing their talents anywhere they are likely to meet a jealous wizard or cleric (in Thay, for example).
More on running a psionics campaign in Faerûn: Psionic Campaign
Psionics and Regional Feats
The psion and psionic warrior classes are not favored in any region of Faerûn. Regional feats (and regional bonus equipment) are not available to such characters.
If you decide that you want psionics to play a greater role in your campaign, you can relax these restrictions somewhat. Tashalar, Chult, the Underdark, the Vilhon Reach, and the Gray Dwarf "regions" are more likely than other parts of Faerûn to give rise to psionic characters and might be considered preferred regions for psions and psionic warriors.
Mantles
Ardents and divine minds create their list of potential powers by choosing specific mantles. Once a mantle is selected, the character can choose from the powers of that mantle when selecting a new known power. Each mantle also has an associated granted ability that is available to the divine mind or ardent once the domain is chosen.