Dear Hélène
Are you using ODK Build or are you using excel?
If you are using excel place the questions you want in a group (i.e. use the begin group function); then set the appearance of that group to 'table'. You do this by just putting the word table in the appearance column. You will have to do this to all groups of questions you want to have displayed in table form.
Please note that the size of the device screen will determine how easy the form is to use and the amount of scrolling from side to side will increase proportional to the size of the matrix.
So this is really not good for large tables.
There was a discussion on this some time back when ODK was still on google groups, the link is here: Collect data for a table/matrix
I hope if your table is small my quick solution helps.
Cheers