Sir, you just made my day!
I created a new wallet in Electrum and selected "restore or import keys", pasted the key I got from Core and it worked perfectly!
Thank you so much, I was close to giving up

The only thing I have is this watch-only wallet in Electrum called "default_wallet".
There is one address in it, which is the address I created as a new receiving address in Bitcoin Core.
This address has all my Bitcoins on it.
I have no cold storage or paper wallet or anything. All I have is this default
-wallet.

Sorry for the bump, I'm just really desperate ...

Hi!
I am completely new to everything bitcoin related. I am using Electrum 2.2.
To receive bitcoins from a poker site, I created an address using the Bitcoin Core wallet.
When I realized that synchronizing takes forever, I downloaded Electrum and used the address from Bitcoin Core
to create my first Electrum wallet.
Everything worked and I received my poker money a few hours later.
Now I want to deposit those coins to my Neteller account.
But appearently the wallet I created in Electrum is a watching only wallet.
I don't know how to sign/verify my payments so when I try to pay to Neteller, the invoice just expires and nothing happens.
How do I get my coins to a wallet that I can make payments with? Or can I make payments with a watch only wallet?

Thanks in advance,
seiesos