DS Lites are incompatible with WPA2 encryption; there's no way to get a field to enter the encryption key because it wasn't designed to work with WPA2 at all.
As Gligar13 suggests, you'll either need to find a
Nintendo Wi-Fi USB Adapter (these are no longer available from Nintendo directly, and I don't believe many retailers would have them in stock, either), or set the encryption on your network back down to WEP.
And if you still play a Gen IV game at all and want to go online with it, even if you did have a DSi, you wouldn't actually be able to use a WPA2 connection. Black and White are the only Pokemon games currently able to do this, and even then it has to be in tandem with one of the handhelds (DSi, DSi XL, or 3DS) that support WPA2.
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