Journey data is the information about a contact that is brought in
when the contact enters a journey. It’s like a snapshot of the
contact’s attribute data at the moment the contact is added to the
Source : https://help.salesforce.com/articleView?id=mc_bp_journey_entry_control_best_practices.htm&type=5
In other words, when you are at the entry point level, the Contact Data are the same as the Journey Data. That's why you can't select "Journey data" in the filter part of the data extension entry point.
To achieve what you want, you have two options :
Option 1 - If you want to filter in the data extension entry point :
-> In order to filter on your data extension at this step, you need to link your data extension in the data designer. You can find here the steps for doing that.
-> This is the reason why you can see your Syncrhonized Data Extension : when the MC Connect is set up, the synchronized data extensions are automatically linked in the data designer.
Option 2 - Use the Journey Data :
-> In order to use a Journey data, you need to create a desicion split. What you can do is to not filter the contacts at the entry point level, but add a decision split as the first step of your Journey.
You will then be able to use the Journey data :
From my side, I think the option 1 should be the one advice by Salesforce. I think it's better for the KPI analysis of the Journey.
With the option 2, every contact that are in your Sync. Data Extension will enter to the Journey and the majority of these contacts will probably go the reminder part of the first decision split.
I hope it helps you !