To 'Grab Active Listing' the listing must be ACTIVE on Trulia!
If not, see how to 'Create New Listing' or 'Import from Datafeed' as appropriate.
On the 'Listings' dashboard click the 'Grab Active Listing' button as shown below and then enter the Zip Code and 'MLS/Source ID' as they are on Trulia and then click the 'Grab' button.
This can take 15-20 seconds. DO NOT CLICK the grab button more than once or you will get the listing duplicated in your account.
If the listing is NOT your listing, but you are borrowing it to hold an open house, market on Facebook using Listing Landing Pages, etc, then check 'No' for 'This is my Listing'.
If you are not able to get this to work, click 'Contact Support' and ask us to do it for you and include the link to the listing on Trulia.