Using coffee ground in the store just doesnt work for home brewed espresso. You must have a good quality grinder and grind immediately prior to brewing.
The grind you use will change from day to day and even through-out the same day to adjust for changes in temp and humidity as well as the age of the beans.
Ground coffee goes stale with-in minutes. If you want to enjoy good espressos at home youll need to invest in a grinder capable of producing a good espresso grind.
You can produce acceptable espresso with mediocre brewing equipment, but not with-out a good grinder. Youll find yourself chasing your tail and constantly dissapointed if you dont have a good grinder at home. With-out a proper grind even the best of brewing equipment is doomed to produce substandard espresso.
By all means get rid of that plastic tamper and buy one thats properly sized for your portafilter.
Java "Its all about the grind" phile