Hi Eduardo. Welcome to the forum!
You can mark an entry as income using the tag #rct. If the entry is in account table, you also need the tag #trf (transfer) or #dp (deposit), otherwise the the value will be added in the expenses as if it was a refund. See example below:
day description value tags
01 Income 1234.56 #rct #trf
The value must be positive, and the sum of all values with the tag #rct is also displayed in the Summary sheet.
Let me know if you have any further questions.