It turns out this is a bug (for Path version 3.1 and earlier) – it will be fixed in the next release.
The problem is that if the value for the Location field of the Deterministic Transitions ends with zero then the import from Excel fails. As a workaround you can change the value for any Locations that end in zero to something else and the import will work. For example:
1. Highlight the Location column in Excel
2. Find and replace 10 with 25
3. Find and replace 20 with 26
Once the values are imported into Path you can change them back again if you like.
Alternatively you can choose to avoid placing any boxes in columns 10 and 20 before you export your Deterministic Transitions.