Nope, the column of ones is fine (although not necessary for you to specify, as the toolbox will automatically create one if not present already).

For some reason, the toolbox thinks that the design matrix has more columns than rows, which is why I was wondering about the size of the two variables.

After you import the design matrix, are the dimensions of the participants_scores variable 67x1?  I'm wondering if the importing process flips the dimensions of the variables.