Yaduvanshi, what version of DEVTEST and ALM are you using? can you paste the entire stack trace of the error message you are getting?
Wanted to verify couple of things from you:
Do you have 32-bit install of DEVTEST? ALM is compatible with only 32-bit Devtest.
Have you installed Add-in's from ALM:
HP ALM Connectivity
HP ALM Client Registration
Thanks
Shiney.