I think it will just charge that as you start to use it each day
This page is a bit clearer
Pay As You Go Mobile Internet Pricing with Meteor mobile
There is an option to have 250 MB free on top-up by €20, but that's probably more than you need to add
Depending on how many days you're there, the other networks also have some bundles, like €5 for 500 MB on 3, or €10 for 700 MB on O2