Check in Database view for Admin>Security to confirm the user has rights to see Proposals
Check in Database view that Constituent records have a tab for Prospect.  The Prospect Module (SEARCH Module) is an optional module and may not be included with your version of RE or RE NXT.

In NXT go to Control Panel>Security>Security Group
Open a Security Group
Under Lists rights confirm that Access to opportunity lists is marked with Yes.  
If marked No, click Edit rights and change this selection to Yes.