The getting started guide walks through using the checkout wizard to create the eCart checkout pages:
For PayPal Standard, you will only need to enter you merchant ID. The member ID is the only PayPal specific setting in the checkout wizard for Payments standard.
What is the nature of the trouble you are having?
Does the checkout process go to the checkout failure page?
if so, take a look at the "Debugging eCart transaction failed issues" tutorial in the eCart Tutorials section of the eCart support page: