Instacast integrates the import and export of your subscriptions from and to Dropbox. This is done via an OPML file. OPML means Open Processor Markup Language and is basically XML. It contains the titles and links of your podcasts subscriptions. This OPML file can also be exported from iTunes.
Instacast communicates with Dropbox via a folder called "Instacast" that needs to be in your root directory of your Dropbox.

To import subscriptions into Instacast:
Export subscriptions from iTunes or from any other Podcasting application that supports OPML.
Create a folder called "Instacast" inside the root folder of your Dropbox.
Move or copy the OPML subscriptions file into the "Instacast" folder and rename it "Subscriptions.opml".
In Instacast, tap on the action bar button in the lower right corner of the subscriptions view and choose "Import from Dropbox".
Enter your Dropbox username and password if necessary. You only need to enter your username and password once. Instacast stores a token that from then on links to your Dropbox account. To unlink Instacast from your dropbox, see below.
Your podcast subscriptions are imported momentarily.
To export subscriptions from Instacast:
Tap on the action bar button in the lower right corder of the subscriptions view and choose "Export to Dropbox".
Enter your Dropbox username and password if necessary. You only need to enter your username and password once. Instacast stores a token that from then on links to your Dropbox account. To unlink Instacast from your dropbox, see below.
Instacast saves your podcast subscriptions into the OPML file called "Subscriptions.opml" inside the "Instacast" folder on the root level of your Dropbox.
To unlink Instacast from your Dropbox account:
Open the Instacast settings, scroll down and select "Accounts" from the "Sharing" settings group.
Turn on the switch of the "Unlink" item of the "Dropbox" settings group.
Switch to Instacast. Now Instacast is not connected anymore to your Dropbox account and the next time you import or export podcast subscription, you have to enter your username and password again.