You can find your personal Base CRM Access token in Base CRM under User Settings / OAuth / Access Tokens / Add access token inside Base CRM. In NiftyQuoter go to My account / Integrations / Base CRM page and paste your access key from Base CRM there.
Importing clients from Base CRM
Either check the 'Import clients from Base CRM automatically when someone creates a new proposal' checkbox and whenever you create a proposal it will automatically import clients from Base CRM. Or use the 'Import clients from Base CRM' button on the clients page in NiftyQuoter. This process may take a few minutes depending on the number of your contacts. You can reload the page after a few minutes and your clients from Base CRM should be there.