Click on the "Upload" button and choose the file you wish to upload (in .txt or .csv).

Do note that the number of keywords you can upload at a single time is limited by your plan:

After you have added a bunch of keywords to Keywords Explorer (or uploaded a list of keywords), you'll see the Overview section with various diagrams and keywords ideas.
To get stats for your initial keywords input, just click the Metrics tab:

Here you'll see advanced metrics for all your keywords with an option to export this list.
Do note that 1 credit is taken for fetching you advanced metrics for each keyword here.

Related
- Does repeated analysis of the same keyword in Keywords Explorer always consume metric credits?
- Is 'metric credits' In Keywords Explorer a monthly limit?
- How can I remove or rename my keywords lists?