Go to the Query module (menu up -> Queries).
At the top of the list of interest checkboxes, check the type of search to perform (either contacts that are assigned to at least one of the interest checkboxes, or all interest checkboxes).
Then check the area(s) of interest to search.
Finally, click on the "Search" button.
You get a list of contacts assigned to these areas of interest.
Then click on the button "Export to CSV". You then get the list as a CSV file.
Open this file in Excel. The "GSM" column contains the mobile numbers of the selected contacts.
Use this column to send your SMS campaign.