How do I insert a restriction enzyme site (RE site) into a sequence?
Open a DNA Sequence File and Define the Insertion Point

Open a DNA sequence and switch to Sequence view.
Click at the position you wish to insert the RE site, or select 1 or more nucleotides if you wish to replace a region with the RE site.
Add the Restriction enzyme site

Click menu Edit → Insert → Restriction Site....

Type the restriction enzyme site name, or select from the drop-down list of restriction enzyme sites.
Click Insert (or Replace) to add the selected site to your sequence.
View the Newly Added Restriction Enzyme Site

Select the "Show Enzymes" and "Show Colors" buttons in the side toolbar to highlight (in red) the newly added site and DNA sequence.
Save the modified sequence.
If you wish to add multiple sequence elements to an existing sequence then consider using the Design Synthetic Sequence tool