How Do I Use A Network Key?

Well, just the prompt said, you need to enter:

- 5 or 13 ASCII characters (All the numbers, letters and symbols you can find on you keyboard)
OR
-  10 or 26 hexadecimal characters (0 -9, A-F)

Now, you said your key consists of 10 characters. By default, the system takes this key to be ALL HEXADECIMAL CHARACTERS because the length is 10. I believe your key has characters outside the Hex range {0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F}

Confirm this please.

Ok, but I never asked you to paste your WEP key here. All I said is, does the key contain any character(s) outside this set:
{0,1,2,3,4,5,6,7,8,9,A,B,C,D,E,F}

If yes, then the key is invalid because the network/system decodes WEP keys with a character length of 10 as HEXADECIMAL, and any character outside the ones I listed above in braces is not a hex digit but ASCII character.

Check this:
1AD34H4BED is not a valid key because the character "H" is an ASCII character not HEX. As such, the system expects you to enter a key with a length of 5 ASCII characters(10 HEX digits) for a 40bit encrypted network or 13 ASCII characters(26 HEX CHARACTERS) for a 104bit encrypted network.

Got the logic?

By the way, did you assign the key yourself or where did you get it from? If someone gave it to you to give you access to a secured network, then my guess is that he played you! or are you trying out random sequence for the key?