Try this and a little ingenuity Felipe.
https://groups.google.com/forum/#!category-topic/mitappinventortest/LlYPkhP1Bw4
Use a Textbox to input latitude , longitude and parse it to a global variable of coordinates for a Marker or use two TextBoxes..one for latitude, the other longitude.
You can also have the user tap a location on the Map and capture the coordinates but the example does not show that method that might work for you too.
Try some blocks. When you find a solution consider posting them here for others to learn. If you have issues, ask again here...this is easy but requires a lot of blocks and an knowledge of the MIT Maps documentation.l
Happy New Year,
Steve