You can import a well-formed JSON-formatted into the Tester with the Ruletest menu command Testsheet > Import >JSON.

You can quickly generate an input document from an existing Testsheet. Click anywhere in the Input column, and then choose either Export JSON to create a file, or click Export JSON to clipboard.

The export file from the Cargo sample's Cargo.ert Input column is as shown: 
[
    {
        "volume": 10,
        "container": null,
        "weight": 1000
    },
    {
        "volume": 40,
        "container": null,
        "weight": 1000
    },
    {
        "volume": 20,
        "container": null,
        "weight": 30000
    },
    {
        "volume": 10,
        "container": null,
        "needsRefrigeration": true,
        "weight": 1000
    }
]