X1 does not support Microsoft OneNote by default. However, with OneNote installed on your computer, then you should have a corresponding IFilter automatically installed on your computer which will allow X1 to index (but not preview) OneNote files.
Therefore, you should be able to index the content of OneNote files using these steps:
1. In X1, go to Tools > Options > Files > Select Files to Scan > Specify File Types
2. In the "Additionally index all files with these extensions:" dialog, enter: .ONE
4. Update your file index manually by clicking the "Index Files Now" button
Once your index is updated, you should be able to perform searches against your OneNote documents.
Please note: that this IFilter only supports text extraction (searching) and you will not be able to preview the OneNote documents.