you should use import private keys, although it may be a better idea to sweep those keys instead of just importing them.
from the menu on top click on Wallet > Private Keys > [there are three options here, sweep, import, export which are self explanatory]
if you just want to import them, click import, read the warning and click OK, then enter the private keys separate by enter or open the file containing them.
p.s. the wallet that you want to import those keys should be a wallet without seed.
here is the Documentation link:
http://docs.electrum.org/en/latest/faq.html#can-i-import-private-keys-from-other-bitcoin-clients