Ali, do you have all the SL2/SL3 requirements? Meaning .Net 3.5 SP1 and VS2008 SP1, SL SDK and SL VStools?
These must be in place before you can open any SL project whether from Telerik or elsewhere.
Secondly, do you have Telerik RADcontrols intstalled? After installation, did the installation opened the demo successfully?
Thirdly, if the answer to above is yes, did you try to open the solution?
..Ben