when developing a custom SPD 2010 workflow activity using Visual Studio , sometimes you can get an error saying “"Cannot find property named PROPERTYNAME on activity type ACTIVITYNAME" when trying to assign a parameter to a workflow activity. This can especially occur if the parameter is newly added to the activity (I.e. your previous version of the activity did not have this parameter or it had it with a different name.
a solution might be to close the SPD and clear the cache at:
Windows 7:
C:\Users\<username>\AppData\Local\Microsoft\WebsiteCache
Delete all the files and folders in this folder
C:\Users\<username>\AppData\Roaming\Microsoft\SharePoint Designer
Delete all the files and folders in this folder
Emad's Blog 