Taking Discard Pile with One Natural Card in Hand
The Rules of Canasta clearly state:

TAKING THE DISCARD PILE
When the discard pile is not frozen against his side, a player may take it:

a) with a natural pair matching the top card as above; or
b) by melding the top card with one matching natural card and one wild card from his hand; or
c) by adding the top card to a meld he already has on the table.

Having taken and melded the top discard as described, the player takes the rest of the pile into his hand and may then meld some or all of the additional cards as he pleases.
....

Thus it is not allowed to use a card deeper in the discard pile as part of the meld enabling a take of the discard pile.