I purchased some btc in 2013 and stored them in a multibit wallet. I've since tried to import the wallet to Electrum, but it's a watching-only wallet. I can see the original transactions, however I can't do anything with them. So I'm led to believe the key I have is just the wallet address?
Correct. If you have created a "watching-only" wallet in Electrum, then what you entered was your wallet address... and not the private key.
I don't have the seed, and I can't find any files on my old machine relating to wallet.dat etc.
Multibit "Classic" never used seeds. It only generated random private keys.
MultiBit "HD" did use seeds.
Are they lost, or can they be redeemed through other means?
You will need one of the following:
- The private keys
- The 12/24 word seed phrase (for HD wallets)
- The wallet file (and any wallet file password)
Without at least one of those, you will not be able to recover your coins.
File types for MultiBit that can be useful to recover private keys or seeds:
[walletname].wallet
[walletname].wallet.cipher
[walletname].key
mbhd.wallet.aes