Hi jamaz,
You can delete the file /var/cache/pacman/db.lck when using PackageKit.
GNOME PackageKit is still under heavy development with the pacman backend, we recommend that you use pacman via CLI instead. Here are the guidelines on the basics of pacman.
To sync the system type,
pacman -Syu
To refresh the package database type,
pacman -Syy
To install a new package type,
pacman -S packagename
To remove a package type,
pacman -Rd packagename
To clean previous cache files type,
pacman -Scc
To search a package type,
pacman -Ss keyword
To display the contents of a package type,
pacman -Ql packagename
To display the information of a package type,
pacman -Qi packagename
Pacman uses binary installation, additionally you can use yaourt for source code installation which extract files from SCM repositories.
To search a package type,
yaourt packagename