Goto: Tools -> Preferences -> Fees
Tick "Dynamic Fees" and also "Edit fees manually"... and I'd recommend setting the "Replace-By-Fee" option to "Always":
When you go to send a transaction, it should enable you to use the slider, but also see the "absolute" fee:
For the record, I highly recommend you don't just type a number in unless you are sure you have calculated it correctly. Otherwise you are likely to end up setting a fee that is too low and your transaction will get stuck.