How can I reconstruct exported oscilloscope data?
The information you need is almost all right there at the top of the data dump:


Sample rate
Full scale voltage
Ground level


With one piece missing : it's presumably an 8-bit ADC, i.e. it counts from 0 to 255, which you can infer from GND being approximately half way (127). But confirm this from the Velleman datasheet.

So, count the number of samples N between peaks (e.g. positive peaks) or better, positive-going "zero" crossings (where "zero" = 127, the GND level).

Frequency = Sample Rate/N.

Voltage of any given sample S is
(S - 127) * Full Scale/256.

Amplitude is simply the peak voltage, best calculated as
( Voltage (positive peak) - Voltage (negative peak) ) / 2
which accounts for any DC offsets. You have a spreadsheet right there - it's ideal for the job.