First and foremost, there is no need to worry. This mixed case feature is a way of identifying typos in the address. Ethereum wallet addresses are in hex [0-9 A-F]*. While the address itself is case-insensitive ("A" is the same as "a" to the network), the case sensitivity is used as an optional checksum. It was built as an after-thought to an addressing scheme that lacked basic checksum validation.
Keep in mind that all Ethereum addresses are NOT case sensitive. Enter the address in which the address is not case sensitive - though both will work!