To do this go to "file" and select "import session". This will open up the external file import dialog window. Browse to the file that you want to import and click the import button. This will open up a second dialog box. From here you have the ability to assign some of the data columns to the appropriate data role such as velocity etc. The software will do its best to assign the columns based on the column headers. So if you have GPS data with velocity labeled "Velocity" the software will recognize this and assign that column of data to the velocity data role. The data roles are then used by the software for specific functions such as rate calculation or report generation. The only critical column is the time column. It should be the first column in your spreadsheet and labeled "time". Check that the time format is correct for your 50Hz data. Once successfully imported, you will need to place an in marker at the start of the file where you want the files to be merged from. Then close the file. Open the MMS session that you want to merge your 50Hz data with. Again set the in marker at the point where you wish to merge the files and close the file. You can then "merge" the two files by going to "file" "open". Hold down control to select the two files that you want to merge. Click the merge button. You then have a small dialog window appear. For example, I use to merge peach and MMS data routinely. Select "new session" and 'in-marker" and then press merge. You should now have a new session will all the data streams synchronized. Remember that if the session is very long (30mins plus) the data could start to look out of sync. This is due to drift between the clocks on the measurement systems not the software.