Usually, the OK button is disabled, if any data set has conflicting changes (was edited locally and was already changed on the repository). This is indicated by a red double arrow on the data set. You will need to manually merge these conflicts by double-clicking on the data set in question. A dialog will open, displaying the local data set on the left and the remote data set on the right. You can then copy the changes that you want to take from right to left, and keep changes that you made untouched. After clicking 'Mark as merged' (for all conflicts) you will be able to continue the fetch process.