Hi Felipe,
Only a subset of all PSM enabled data types are available to be used in an extension BO. The Repository Explorer does not document this. The easiest way to know what is available is to use the code completion function (ctrl + space) to see what data types are available.
One alternative in this situation is to use the Decimal data type, while setting the number of places after the decimal to zero. Something like this:
[Decimal (x,0)] element MyDecimal : DecimalValue;
Where x is anything up to 29, the total number of digits allowed. Keep in mind that negative values will use one place for the negative sign. This means only 28 actual digits for negative numbers.
Cheers!
Nick