If you want to insert an Alt Fragment to your diagram in Software Ideas Modeler, you can do it this way:
Choose Alt Fragment Tool
Choose the Alt Fragment tool from toolbox.
Insert Alt Fragment
Draw the Alt fragment - click (and hold) on the position where you want to be the top left corner of the fragment and move to the right bottom corner of your fragment.
Add Interaction Operand
Click on Add Interaction Operand button in the context bar below the fragment. If you want to add two operands, click twice (of course, you can click as many times as you want).
Move the mouse cursor to the dash line between interaction operands, press the left mouse button and drag the line to the desired position.
Set Guard Condition
Double click on interaction operand and Properties dialog will show. Then open the Interaction Operand tab, where you can set the guard condition for the interaction operand.
Add Alternative Call
Using Send Action Tool (or any action tool you want) add the alternative message to the second interaction operand.