to be able to edit the power store using eCart, you will need to make sure that the Dreamweaver site is defined correctly.
The Power Store Files must be at the root of the Dreamweaver Site.
Also, make sure that the WA_eCart folder is not set to read only. the files in the eCart folder must allow read and write access.
you say it added a wrong tax code?
how did you set the rule?
How was the code wrong?
are you intending to replace the shipping rules that are already in eCart?
If so, you should delete those rules, then add your new ones.