To start annotating a Dataset you need to configure your labels, to do so let's go to your "All Dataset"
And click on the Dataset that you want to annotate, you will arrive in your dataset's version list
Then choose the appropriate version of the dataset you want and you will arrive on the dataset overview page.
You can now access the labels configurator
If another version of your dataset exists, you can choose to import classes from an older version or create some brand new classes to annotate