Which browser are you using?
In Firefox, click the drop down arrow next to the search engine icon and choose "manage search engines"
On the window that comes up, click "Get more search engines." This will open the Add-ons page for Firefox.
Enter Dogpile in the search box and then click Add To Firefox when it displays Dogpile.
Back at the main Firefox window, again click the drop down arrow next to search engines and pick whichever you would like to be the default. It will stay at this setting until you change it to anotherone.
Problems with Bing:
In addition to the normal search engine options, Microsoft has partnered Bing with an evil, scum-sucking, bottom feading browser hijacker named Conduit. Conduit "sponsors" many sources of freeware and shareware and has their crapware installed by default when you install many programs downloaded from places such as Softpedia, Cnet, Tucows, etc. During the program installation, there will be one of the screens where you hit "Next" to continue the installation. There will be a checkbox in the windows which is already checked and in rather plain, unremarkable text will be something to the effect of Install Conduit (or some other name) program/search engine/internet saftey feature/Conduit Search Protect or some other bull. Most people unthinkingly hit "Next" without paying attention and Microsoft and Conduit are counting on this. Once installed, it circumvents your preferences, setting (usually) Bing as the default payge when you open a new tab. It may also circumvent your settings for default search engines and other such things.
To get rid of this malware is not an easy task. For one, it is something you had to agree to be installed, even though you didn't realize it, so anti-malware software does not recognize it as anything it shoud deal with.
Conduit hooks itself into the operating system at a very deep level - almost as deep as a root-kit virus and more deeply than most viruses can. To get rid of it is a several step process. This site has very clear step by step instructions on how to go about this. Be sure to follow every step in order to clear this infection completely.
Not included in their steps but of equal importance are a couple of final steps - cleaning the registry and deleting files/folders not removed by the above.
There are settings for Conduit and Bing which the above steps do not necessarily remove. Getting rid of these is a step which is a bit tedious, but important.
Open Registry Editor (Click Start -> type Regedit <enter>
*VERY IMPORTANT* improper editing of the registry can render your Windows inoperable. Please make a backup copy of the registry before making any changes: Click File -> Export -> And at the bottom where it says Export Range, select "All" Specify a back-up name such as reg_backup_todaysdate and hit save.
In the registry, click on the line which says "HKEY_CURRENT_USER (You can skip HKEY_CLASSES_ROOT - There are scores of entries there however, you can leave them there with impunity and it would take you hours to get rid of them all.)
At the top click Edit -> Find and enter Conduit and then search.
Delete each key which comes up with Conduit. After deleting a key, press F3 to search to the next entry.
When this is complete, repeat the search by going up to the top and clicking on HKEY_CURRENT_USER again and then searching for BING and deleting those entries.
Finally, using Windows Explorer, search your computer for files and folders containing Conduit and Bing and delete those.
You can now reboot and be comfortable knowing that Conduit and Bing are truly gone.
Now that you see how much work this is, PLEASE make sure that you read the screens which come up during software installations BEFORE you click Next to make sure you know to what you are agreeing.