First you need to create an account to use the bug tracker by clicking here : http://forge.prestashop.com/secure/Signup!default.jspa|http://forge.prestashop.com/secure/Signup!default.jspa
Fill all field and validate your account.
When you validate the form Click here to log in.
You can now create your first issue :
1 : click on "Create Issue"
2 : choose the project : PrestaShop version 1.4 or 1.5, Prestashop Native Modules & Themes
3 : choose the issue type : bug or improvement
4 : Submit
On the next screen fill all informations to create your issue.